Validation of Draft Document using Transaction Notification

Hi,

is it possible to validate a PO Draft Document upon creation using Transaction Notification.The reason I want to do this is because in our system we have an Approval Process for Purchase Orders. Now as you know each PO is created as a Draft document when it is waiting for approval. Now I have a few UDFs on the PO which I must make sure they are filled in before the PO is submitted for approval. Any ideas?