tl;dr: TODO

((If you are unlucky enough to have stumbled into this page, without me having finished it, see this https://github.com/phalasz/forgesteamworksexample))

Look at these classes. They are the important ones - the links of Facepunch and Forge.
https://github.com/phalasz/forgesteamworksexample/blob/master/Assets/ForgeSteamworksNetExample/Scripts/SteamworksMultiplayerMenu.cs
https://github.com/phalasz/forgesteamworksexample/blob/master/Assets/ForgeSteamworksNetExample/Scripts/SteamworksJoinRequestCallbacks.cs
https://github.com/phalasz/forgesteamworksexample/blob/master/Assets/ForgeSteamworksNetExample/Scripts/JoinMenu.cs