Add one more field level under Interaction Area


In Interaction ticket, there are Subcategory & Area under each category. I have created a new field "Category (level 3)" and the associated table, added the corresponding entry in the "incidents" link and it looks good on the form. It works like Category -> Subcategory -> Area -> Category (level 3).

The problem is, for Interaction Subcategory & Interaction Area, user can select them in "1 step" on the ticket (i.e. click the Fill button for Subcategory, select a Subcategory & it will show the child Areas. User does not need to click the Fill button for Area separately). But for the new field, how can I configure it just like that?

Please help, thanks!