Start with the QuickStart Guide, DataBinding chapter, to see if your usage makes sense. Then, see the FAQ for the information you should post if you need help.
If I add the editortype to the DataSourceTextField itself, it works fine. The problem is if I define a DynamicForm and this DataSourceTextField as one of the form’s field (somehow bind together by DateSource), and use the below statement to get the field as a FormItem: