It sounds like the Domino server is configured to use Session authentication. I don't disagree with the previous response. That sounds right if you must use session auth, but it may be easier to use Basic authentication.
For a comparison of session and basic auth see Authenticating Domino REST Service Requests. If the server is configured for session auth, you can still use basic auth for your requests. But you should consider adding web site rules to override session auth for selected URLs.