Patch the rejection reason or rejection question custom field answers on a rejected application without changing its rejected_at timestamp. The {id} path parameter is the rejection detail record id, not the application id — fetch it from GET /v3/rejection_details?application_ids={application_id} first. The application must already be rejected; this endpoint cannot reject an application that is still in process. Only the keys included in the request body are modified; custom_fields replaces the listed rejection question values wholesale.