diff --git a/app/controllers/admin/custom_fields/custom_field_projects_controller.rb b/app/controllers/admin/custom_fields/custom_field_projects_controller.rb
index d392aedde22d..785ba7a0147e 100644
--- a/app/controllers/admin/custom_fields/custom_field_projects_controller.rb
+++ b/app/controllers/admin/custom_fields/custom_field_projects_controller.rb
@@ -29,6 +29,9 @@
 #++
 
 class Admin::CustomFields::CustomFieldProjectsController < ApplicationController
+  include OpTurbo::ComponentStream
+  include OpTurbo::DialogStreamHelper
+
   layout "admin"
 
   model_object CustomField
@@ -36,16 +39,37 @@ class Admin::CustomFields::CustomFieldProjectsController < ApplicationController
   before_action :require_admin
   before_action :find_model_object
 
+  before_action :available_project_custom_fields_query, only: :index
+  before_action :initialize_custom_field_project, only: :new
+  before_action :find_projects_to_activate_for_custom_field, only: :create
+
   menu_item :custom_fields
 
-  def index
-    @available_project_custom_fields_query = ProjectQuery.new(
-      name: "custom-fields-projects-#{@custom_field.id}"
-    ) do |query|
-      query.where(:available_project_custom_fields, "=", [@custom_field.id])
-      query.select(:name)
-      query.order("lft" => "asc")
+  def index; end
+
+  def new
+    respond_with_dialog Admin::CustomFields::CustomFieldProjects::NewCustomFieldProjectsModalComponent.new(
+      custom_field_project_mapping: @custom_field_project,
+      custom_field: @custom_field
+    )
+  end
+
+  def create
+    create_service = ::CustomFields::CustomFieldProjects::BulkCreateService
+                         .new(user: current_user, projects: @projects, custom_field: @custom_field,
+                              include_sub_projects: include_sub_projects?)
+                         .call
+
+    create_service.on_success { render_project_list(url_for_action: :index) }
+
+    create_service.on_failure do
+      update_flash_message_via_turbo_stream(
+        message: join_flash_messages(create_service.errors),
+        full: true, dismiss_scheme: :hide, scheme: :danger
+      )
     end
+
+    respond_to_with_turbo_streams(status: create_service.success? ? :ok : :unprocessable_entity)
   end
 
   def default_breadcrumb; end
@@ -56,8 +80,70 @@ def show_local_breadcrumb
 
   private
 
+  def render_project_list(url_for_action: action_name)
+    update_via_turbo_stream(
+      component: Admin::CustomFields::CustomFieldProjects::TableComponent.new(
+        query: available_project_custom_fields_query,
+        params: { custom_field: @custom_field, url_for_action: }
+      )
+    )
+  end
+
   def find_model_object(object_id = :custom_field_id)
     super
     @custom_field = @object
   end
+
+  def find_projects_to_activate_for_custom_field
+    if (project_ids = params.to_unsafe_h[:custom_fields_project][:project_ids]).present?
+      @projects = Project.find(project_ids)
+    else
+      initialize_custom_field_project
+      @custom_field_project.errors.add(:project_ids, :blank)
+      update_via_turbo_stream(
+        component: Admin::CustomFields::CustomFieldProjects::NewCustomFieldProjectsFormModalComponent.new(
+          custom_field_project_mapping: @custom_field_project,
+          custom_field: @custom_field
+        ),
+        status: :bad_request
+      )
+      respond_with_turbo_streams
+    end
+  rescue ActiveRecord::RecordNotFound
+    update_flash_message_via_turbo_stream message: t(:notice_project_not_found), full: true, dismiss_scheme: :hide,
+                                          scheme: :danger
+    update_project_list_via_turbo_stream
+
+    respond_with_turbo_streams
+  end
+
+  def update_project_list_via_turbo_stream(url_for_action: action_name)
+    update_via_turbo_stream(
+      component: Admin::CustomFields::CustomFieldProjects::TableComponent.new(
+        query: available_project_custom_fields_query,
+        params: { custom_field: @custom_field, url_for_action: }
+      )
+    )
+  end
+
+  def available_project_custom_fields_query
+    @available_project_custom_fields_query = ProjectQuery.new(
+      name: "custom-fields-projects-#{@custom_field.id}"
+    ) do |query|
+      query.where(:available_project_custom_fields, "=", [@custom_field.id])
+      query.select(:name)
+      query.order("lft" => "asc")
+    end
+  end
+
+  def initialize_custom_field_project
+    @custom_field_project = ::CustomFields::CustomFieldProjects::SetAttributesService
+                        .new(user: current_user, model: CustomFieldsProject.new, contract_class: EmptyContract)
+                        .call(custom_field: @custom_field)
+                        .result
+  end
+
+  def include_sub_projects?
+    ActiveRecord::Type::Boolean.new.cast(params.to_unsafe_h[:custom_fields_project][:include_sub_projects])
+  end
 end
diff --git a/app/forms/projects/custom_fields/custom_field_mapping_form.rb b/app/forms/projects/custom_fields/custom_field_mapping_form.rb
index 1f598a730673..f637ea059934 100644
--- a/app/forms/projects/custom_fields/custom_field_mapping_form.rb
+++ b/app/forms/projects/custom_fields/custom_field_mapping_form.rb
@@ -44,7 +44,7 @@ class CustomFieldMappingForm < ApplicationForm
             multiple: true,
             dropdownPosition: "bottom",
             disabledProjects: projects_with_custom_field_mapping,
-            inputName: "project_custom_field_project_mapping[project_ids]"
+            inputName: "#{input_name}[project_ids]"
           }
         )
 
@@ -82,5 +82,9 @@ def projects_with_custom_field_mapping
     def join_table
       @project_mapping.class
     end
+
+    def input_name
+      join_table.model_name.singular
+    end
   end
 end
diff --git a/app/views/admin/custom_fields/custom_field_projects/index.html.erb b/app/views/admin/custom_fields/custom_field_projects/index.html.erb
index 7bf383a31848..b665e8902251 100644
--- a/app/views/admin/custom_fields/custom_field_projects/index.html.erb
+++ b/app/views/admin/custom_fields/custom_field_projects/index.html.erb
@@ -34,6 +34,22 @@ See COPYRIGHT and LICENSE files for more details.
                                                           selected: :custom_field_projects))
 %>
 
+<%=
+  render(Primer::OpenProject::SubHeader.new) do |component|
+    component.with_action_component do
+      render(Primer::Beta::Button.new(
+        scheme: :primary,
+        tag: :a,
+        href: new_custom_field_project_path(@custom_field),
+        data: { controller: "async-dialog" }
+      )) do |button|
+        button.with_leading_visual_icon(icon: 'op-include-projects')
+        I18n.t(:label_add_projects)
+      end
+    end
+  end unless @custom_field.required?
+%>
+
 <%=
   if @custom_field.is_for_all?
     render Primer::Beta::Blankslate.new(border: true) do |component|
diff --git a/config/locales/en.yml b/config/locales/en.yml
index b69c3b9919b4..3913755c241c 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -957,6 +957,10 @@ en:
               minimum: "need to include at least one filter for principal, context or id with the '=' operator."
         custom_field:
           at_least_one_custom_option: "At least one option needs to be available."
+        custom_fields_project:
+          attributes:
+            project_ids:
+              blank: "Please select a project."
         custom_actions:
           only_one_allowed: "(%{name}) only one value is allowed."
           empty: "(%{name}) value can't be empty."
diff --git a/config/routes.rb b/config/routes.rb
index 599deee6f166..223d674cfbec 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -174,7 +174,7 @@
       scope module: :custom_fields do
         resources :projects,
                   controller: "/admin/custom_fields/custom_field_projects",
-                  only: %i[index]
+                  only: %i[index new create]
       end
     end
   end
diff --git a/spec/features/admin/custom_fields/custom_fields_project_spec.rb b/spec/features/admin/custom_fields/custom_fields_project_spec.rb
index fc847d44ca9a..0bd024cf0322 100644
--- a/spec/features/admin/custom_fields/custom_fields_project_spec.rb
+++ b/spec/features/admin/custom_fields/custom_fields_project_spec.rb
@@ -80,6 +80,51 @@
       end
     end
 
+    it "shows an error in the dialog when no project is selected before adding" do
+      create(:project)
+      expect(page).to have_no_css("dialog")
+      click_on "Add projects"
+
+      page.within("dialog") do
+        click_on "Add"
+
+        expect(page).to have_text("Please select a project.")
+      end
+    end
+
+    it "allows linking a project to a custom field" do
+      project = create(:project)
+      subproject = create(:project, parent: project)
+      click_on "Add projects"
+
+      within_test_selector("new-custom-field-projects-modal") do
+        autocompleter = page.find(".op-project-autocompleter")
+        autocompleter.fill_in with: project.name
+
+        expect(page).to have_no_text(archived_project.name)
+
+        find(".ng-option-label", text: project.name).click
+        check "Include sub-projects"
+
+        click_on "Add"
+      end
+
+      expect(page).to have_text(project.name)
+      expect(page).to have_text(subproject.name)
+
+      aggregate_failures "pagination links maintain the correct url" do
+        within ".op-pagination" do
+          pagination_links = page.all(".op-pagination--item-link")
+          expect(pagination_links.size).to be_positive
+
+          pagination_links.each do |pagination_link|
+            uri = URI.parse(pagination_link["href"])
+            expect(uri.path).to eq(custom_field_projects_path(custom_field))
+          end
+        end
+      end
+    end
+
     context "and the project custom field is for all projects" do
       shared_let(:custom_field) { create(:user_custom_field, is_for_all: true) }