Answer
The Purchase Order header itself doesn't carry a real, usable link back to its originating requisition(s) — that connection only exists at the Purchase Order Line level, and a single order's lines can trace back to multiple different requisitions. A read-only custom field using a LISTAGG-style SQL statement (aggregating distinct custom-field values across all requisitions linked through that order's lines) is a workable pattern for surfacing an aggregated view of the requisition-side custom field on the order header, in place of a direct one-to-one reference that doesn't actually exist there.