Hi,
If you need to copy the Customer PO number automatically to PR, there is no standard functionality exists as of my knowledge and , you may go for development. If you are going for development, you may copy the Customer PO number (VBKD-BSTKD) to any of the fields like sales order item text / PR text or even in fields like "Tracking Number" in EBAN. You may check the feasibility with technical team.
If you copy to "Tracking Number" field in PR, you can even track the orders based on tracking number (customer PO number) using standard reports like ME5A, ME2N etc.
Regards,
AKPT