Hello,
You can use of BADI ME_PROCESS_PO_CUST method PROCESS_HEADER.
Also check user exit M06E0004 Changes to communication structure for release purch. doc.
M06E0005 Role determination for release of purchasing documents
which will trigger while saving the PO.
Please refer Triggering of release strategy for a Purchasing document – a developmental overview
Best Regards,
R.Brahmankar