http://www.micklerandassociates.com/blog/set-remote-desktop-licensing-mode-on-a-windows-2012-server

The procedure for assigning the licensing mode is to open Server Manager, Remote Desktop Services, and select Overview; from there, a GUI is supposed to make the licensing process relatively easy.

However, when I attempted to do this, Windows reported:

"You are current logged on as local administrator on the computer. You must be logged on as a domain user to manage servers and collections."

This message prevents you from seeing the handy UI.

Regrettably, this is Microsoft-speak for "We never intended anyone to run RDC/Terminal Services on a stand-alone server not a part of a domain, duh", which is asinine, but yes, there is a work-around.

1. Open GPEDIT.MSC.

2. Browse to the following folder: Computer Configuration\ Administrative Templates\ Windows Components\ Remote Desktop Services\ Remote Desktop Session Host\ Licensing

3. Enable the key: Use the specified Remote Desktop license servers. Declare your server. Save.

4. Enable the key: Set the Remote Desktop licensing mode. Declare your licensing mode. Save

5. Shell to DOS with Admin privileges. Do a "gpupdate /force" on the box.

When you run your licensing diagnosis again, you'll find that the errors have gone away and you've successfully assigned the licensing model to the server.