Exit in BAPI_REQUISITION_CREATE

Hello experts!

Well, I have a very basic doubt...

I would like to create a code in an Exit in BAPI_REQUISITION_CREATE (when the shopping cart is generating a requisition in R3 side). I think to use the EXIT_SAPLEBND_004.

In some conditions I would like to "record" some messages, so that I can check later in case of errors. How can I do this?

Thanks a lot!

Cristiana