Campaign API 400 Error mapping Zendesk lists to GC Outbound

What’s the best way to map our existing Zendesk ticket lists to Genesys Cloud Outbound campaigns via the API? We are hitting a 400 Bad Request on the /api/v2/outbound/campaigns endpoint with the message ‘Invalid contact list format’. The JSON structure matches the docs but fails validation. Since we are migrating from Zendesk, is there a specific field mapping requirement for the contact list that differs from the standard GC schema? We need to replicate the old Zendesk view logic exactly.