Dunno then.
RDP in Win 8.1 (titled "Access RemoteApp and desktops" from Control Panel) asks for a URL or an email address, it will not take anything else.
The help text on the setup screen specifies this and shows an example which is https:// (though it ends with a .aspx filename). Try to use http and you are told it must be https.
Everything I've thought to try ends the same way:
The remote computer cannot be authenticated due to problems with its security certificate.
Security certificate problems might indicate an attempt to fool you or intercept any data you send to the remote computer.
Windows cannot continue setting up this connection. Contact your workplace administrator for assistance.
This was just an exercise to see if I could get it working, really, not something I "must have", but figured it was something I had simply overlooked. Any help appreciated, but don't lose sleep over it, lol.