screen validation for FB70

Hello,

I am working on a requirement where I need to put some validation on the FB70 screen. So here is the requirement:

When a user posts/parks the document using FB70 a validation needs to happen to make sure that REF1 and REF2 fields of BSEG table are not blank, my problem is these fields are not on the FB70 screen but these are on one of the sub screen, so can you please help me with this requirement?

I tried to put some validation on teh sub screen and it's working fine but the problem is if the user doesn't go to that particular sub screen that validation doesn't work (as expected) ...now what I want is as soon as user tries to park/post a document and fields REF1 and REF2 are blank then the validation should happen without going to the sub screen,

any help is appreciated.

Thanks,