call another form when select a field, possible ?

In 11.5.10.2.  have a user requirement like this.

I have a custom form have 5 fields.  One of the field called item category.  When user tab/click on the item category field, it should pop up a 2nd form to allow user to select ( LOV ) segment1, segment 2 and segment3.  When user click OK in the 2nd form after enter values in segments, the segment1.segment2.segment3 value will be populated back to the item category field in 1st form.

Of course, we all are thinking... create a DFF in your first form.  That it!   But life is not that easy.

User ALSO wants to have segment 3 as optional.  In the other word.  if user only enter segment1 and segment 2 without segment 3 ( blank ), they are still valid combination ( or I should say.. should not be validate against item category DFF combination ) and require to populate back to the 1st form.  The issue I have to call The DFF in the 1st form... DFF won't allow you exit the 2nd/DFF form until you provide a validation combination.  NULL in segment3 is not a valid  combination ( e.g.  system.board.wire is a validation combination.  but syste.board.NULL is