From https://resonite.com
Dive into a brand new digital universe with infinite possibilities, socialize and hang out with people around the world or to build, create and develop anything.
To use this egg, you will need a beta code and a Steam account. Currently the beta code can only acquired by going to the Resonite Patreon (https://www.patreon.com/Resonite), and subscribing to the "Discoverer" level. Then you will be able to message the Resonite bot in-game with "/headlessCode", and receive the beta code. The account will also need to have Resonite in its library. You can do this by running app_license_request 2519830 in steamcmd if you have never installed Resonite on this Steam account.
For more information related to Headless configuration, ask in the official Resonite Discord: https://discord.gg/resonite
This game does not require any port forwarding, but instead uses UDP NAT hole punching (and/or relay) on a random port. You can force a certain port in the config for direct connections with the forcePort directive, but most communcation is done via NAT hole punch using LiteNetLib (LNL).