IF you are unsure or don't have a clear spec, I would use something
like fiddler2 to observe 'normal' interaction by making manual
requests using a browser, or browser with a rest-testing plugin. See
how the session ID is passed back and forth. Is it a header value,
part of the querystring or something else? there's more than one way
to skin that cat.

Then emulate what the site expects. it's easy to specify header
values or querystring values