BADI for ME21N.

HI Excepts,

im new to badi and user exit..! My requirement is to create the badi or user exit for ME21N.

My Requirement is while creating PO in me21n.!

1. PO Can't be created manually.
2. PO Created only with Reference of PR.
3. IF user Click -->SAVE  without PR , Error Message will Trigger.

Please Share Screenshot,because i'm new to sap-abap...!

thanks,
Akash