I.e. with 2amigos/yii2-ckeditor-widget
In your view
use dosamigos\ckeditor\CKEditor;
use nickdenry\ckeditorRoxyFileman\RoxyFileManager;
And then
$form->field($model, 'body')->widget(CKEditor::className(), [
'preset' => 'full',
'clientOptions' => RoxyFileManager::attach([/* another ckeditor options */]),
]);