Setting default in descriptive flexfield

I have a descriptive flexfield on the AP Invoice header that has a required segment that I want to default based on the supplier type.  It will be a certain value the majority of the time so i was hoping to default it for the majority of the invoices.  i want to set the segment to a default for a specific supplier type but leave it null for other supplier types.  I need the user to populate it for the other supplier types.  Does anyone know how to do this?