How to generate a warning message on AP Invoice header page during data entry

Summary:

How to customize a page and conditionally generate a warning (or notification) message on AP Invoice header page during data entry?

Content (please ensure you mask any confidential information):

Goal is to make sure that AP invoice header data is populated correctly.
Customization should check values for business unit and paygroup.

If BU equals 'BU_Value1' and paygroup equals 'PG_Value1', then value in AP Invoice header DFF should be evaluated. If this DFF value is null or equals 'DFF_Value1', then produce warning message. If DFF value equals 'DFF_Value2', then do nothing.

Version (include the version you are using, if applicable):