How to use PO Buyer in invoice approval Spreadsheet as the Starting Participant

We are using Approval Spreadsheet for Invoice Approvals and for PO matched invoices, starting participant needs to be PO Buyer, We tried using Invoice Header.PO Buyer or Invoice Header Po Buyer but it is not working out and giving error that this user does not exist. Any solution how can we enter Po Buyer as Starting participant in workflow spreadsheet for Invoice?