Hi,
You may maintain the customer PO number in sales order item text, which can be copied to PR / PO item. You may check the below steps:
1. Maintain the customer PO number in texts like "Material sales text" in sales order item level - texts tab.
2. Define text linkage for PR to pull the text from sales order in the path: OLME - Purchase Requisition - Texts for Purchase Requisitions - Define Copying Rules.
3. Similarly, maintain PR to PO text linkage to copy the text from PR to PO. Refer the doc: MM:Define copy rules for PR texts
4. Else, maintain the customer PO number in the fields like "requisitioner" or Tracking Number (In schedule lines tab - Purchase requisition button), which will be copied to PR & PO.
5. Modify the PO print form to include the customer PO number from respective fields.
6. Inform the vendor to include the "customer" PO number in the delivery note.
Regards,
AKPT