Miro user-exit for vendor code/purchase order

Hi People:

I need your help for a (I think) simple question. I am looking for an user-exit for TX. Miro.

When an user enter in Miro the purchase order, we need to get the vendor code, and show a message (depending on some information in master data vendor) before post the document.

Do you know a valid user-exit for this purpose?

Thanks in advance for your time.

Best regards.