curl -X POST "https://api.cartesia.ai/agents/webhooks" \
  -H "X-API-Key: your-api-key" \
  -H "Cartesia-Version: 2026-03-01" \
  -H "Content-Type: application/json" \
  -d '{
    "url": "https://your-server.example/webhooks/cartesia",
    "secret": "YOUR_WEBHOOK_SECRET"
  }'

{
  "id": "<string>",
  "url": "<string>",
  "secret": "<string>",
  "created_at": "2023-11-07T05:31:56Z",
  "updated_at": "2023-11-07T05:31:56Z",
  "display_name": "<string>"
}