The default implementation of the HTTP server class only handles requests to existing files below the HTTP root directory. This means that only static content on the hard disk can be processed.
But often you must process virtual content which is derived from volatile data in the memory of some application. You can extend the functionality in several ways. One of them might be to derive a new class and handle everything "manually".
But instead of this you should use the helper classes that come with ulxmlrpcpp: handlers for HTTP requests and for processing form data and html content.