API ReferenceAudienceCustom fields
List custom contact field definitions
curl -X GET "https://example.com/api/v1/custom-fields"{ "data": [ { "id": "550e8400-e29b-41d4-a716-446655440000", "workspaceId": "550e8400-e29b-41d4-a716-446655440000", "key": "plan", "label": "Plan", "fieldType": "text", "subscriberEditable": false, "createdAt": "2026-01-01T00:00:00.000Z" } ]}