Though verification of a blank Reference (PO) is not available, there is an option to check for duplicate references. You could try putting in some value in the Entry template for the Reference field like **ENTER** so that it's visible. And have some dummy order already posted with that reference. So every time an order is created, this value gets put into the reference field and if it isn't changed, then the warning will appear and force entry to the reference.
_________________________
Regards,
Softrak Tech Support