Outbound Campaign API 400 Error mapping Zendesk lists to GC Outbound

Attempting to migrate Zendesk ticket lists to Genesys Cloud Outbound campaigns via the API. The process fails with a 400 Bad Request on the /api/v2/outbound/campaigns endpoint. The JSON payload mirrors the Zendesk list structure but GC rejects the listId reference. Why does this setting cause a validation failure when mapping legacy Zendesk data to GC Outbound?