Links a Qonversion anonymous user to an external identity ID.
If user_id is null, a new user is created and linked.
This is the mechanism for matching Qonversion anonymous users with your own user IDs.
Bearer authentication using the project Secret Key (prefixed with sk_, or test_sk_ for sandbox). All v4 public endpoints require the Secret Key — see Authentication. Never expose the Secret Key in client-side code.
Idempotency key for safe retries. Same key returns the original response.
Identity created successfully.