Default Distribution Account in Invoice Distribution Screen

Hi!

I need some suggestions on a requirement we have here.  We are using Apps R12.0.6.  We have several Operating Units.  While entering Invoice Distribution, users want to default Company Segment in the Accounting Flexfield based on the Operating Unit they selected on the Invoice Header level.  As an example, if they select Operating Unit 'AAA' on the header, Company should be defaulted to '01'.  But, if they select Operating Unit 'BBB' on the header, Company should be defaulted to '02' and so on.  My issue is, I can not personalize or do anything with the Accounting Flexfield pop-up screen on