Passing a parameter to a custom OAF screen from the Requisition demand workbench "requisition lines

We are developing a custom OAF screen, which will be linked to a requisition through the requisition_header_id.

Through using personalizations we have created a button within the "Requisition Lines table" on the demand workbench screen within the Buyer Work Centre. There is a button per each row returned in the table.

When we invoke this screen we need to pass the req_header_id for the requisition on the table row against which the button was clicked.  I cannot find any view object under this seeded OAF table to use, please can anyone advise how to pass a parameter when invoked against a record in a table?