(Moving to this forum, since the question has nothing to do with Swing. Please
CarefullyChooseOneForum in the future.)
No, it has nothing to do with RMI. The mechanics depend entirely on the provider you have signed up with. They should have a section on their web site where you can find the details, maybe even sample code.
A 400 response code means that the request was not correct - you may have missed a parameter, or the request was otherwise malformed. Again, the provider should provide you with the details about how to access their service.
I strongly advise to use a proper HTTP client library like
Apache HttpClient that does all the nitty gritty stuff for you.