Hello Dany,
Which SRM version do you have? The Badi you must implement is BBP_CREATE_BE_PO_NEW BADI not this BBP_ECS_PO_OUT_BADI, because you are working under a classic scenario deployment.
In the BBP_CREATE_BE_PO_NEW implementation you must configure the methol FILL_PO_INTERFACE1 (which is related to BAPI_PO_CREATE1) to enrich the output fields CS_PO1_DOCUMENT-IT_POTEXTITEM or CS_PO1_DOCUMENT-IT_POTEXTHEADER.
The fields completed are the same of ERP, sure ABAP team knows how to do it after you specify this.
Hope it helps,
Reward points if usefull.
SG.