An incredibly simple P2P chat message project using Mirror and Unity 3D.
Open the project in Unity, configure the Network Manager game object to reflect your preferred connection settings, and let'er rip.
This is basic; there is no user list or anything of the sort. I may make this more complicated in the future, but it compiles and runs as headless on Linux, and runs well on x64 Windows builds - so I'd call it a successful test.