Hi,
Considering you have already designed Release Procedure for Purchase Requisition.For Header level release in Purchase Requisition, use following path:
SPRO -> Materials Management -> Purchasing -> Purchase Requisition -> Define Document Types
Now check to select check box of OvRelPRel(Overall release of purchase requisitions)for Purchase Requisition Document Types which are subjected Release Procedure.
Go for creation of new Purchase Requisition with Purchase Requisition document type and see how release strategy triggering!
Regards,
Biju K