Field Status Variant

Hi experts;

Case:

on the F-47 I need to set the field of Purch. Doc. as Required to force the user to enter the P.O. number for the vendor I need to make down payment for him/her.

I checked the field status but that field is not between the fields where I can control its status to be (Req., Optional or Suppressed).

So can someone guide how to set a field on a screen for Req. Entry if it is not between the fields in fld. status?