Block invoice posting if Custom field is not populated

Hello everyone,

I have a few custom fields on the SIH0 screen.  If these fields are not set by the user I would like to add a message box (Yes / No) when they post the invoice.  Basically to inform the user that they are empty and if they continue to post the invoice will not be picked up by our custom application for processing.  I tried my best to find an entry point or something that will allow me to do this but I'm stuck unfortunately.

If anyone has any ideas it would be greatly appreciated.

Thanks a lot,

Mike