To open forms, use the OpenForm global context method (when using the 1C: Enterprise 8.2 platform version and earlier versions, also use OpenFormModal). An alternative method, using the GetForm method, is not recommended.
Procedure Test()
Doc = Documents.PlanOperation.CreateDocument();
Form = Doc.GetForm("DocumentForm"); // here
EndProcedure
Procedure Test2()
Form = GetForm("CommonForms.MyForm");
EndProcedure
Source: Development standards (RU)