curl -X POST "https://api.joinformal.com/core.v1.UserService/CreateUserExternalId" \
  -H "X-API-KEY: YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "example": "value"
  }'

{
  "externalId": {
    "id": "<string>",
    "externalId": "<string>",
    "userId": "<string>",
    "appId": "<string>",
    "description": "<string>",
    "createdAt": "2023-11-07T05:31:56Z",
    "updatedAt": "2023-11-07T05:31:56Z"
  }
}