USK supports Photon Multiplayer. To use it, import PUN2 from the Asset Store.
Then, enter an ID of your project in the Photon settings window [Window -> Photon Unity Networking -> PUN Wizard -> Setup Project].
Now you can use multiplayer mode. Best to start by adjusting the Lobby - create game modes, add characters, maps, and weapons in there:
Then you should create game rooms and add some elements in them:
To adjust all multiplayer UI (the lobby, rooms, match stats, etc.) open the UI Manager and follow the instructions there:
Also, see the video on complete creation of a multiplayer game: