You can use IXMLHttpRequest2 http://msdn.microsoft.com/en-us/library/windows/desktop/hh831163(v=vs.85).aspx and manually build the request.
You can also use a C# WinMD component (which is what I would choose if it is a complicated WebService) and add a reference to it in your CPP project and make your calls through that.
http://blogs.msdn.com/b/piyushjo/archive/2011/09/22/wcf-in-win8-metro-styled-apps-absolutely-supported.aspx
-Jeff
Jeff Sanders (MSFT)
