From https://neos.com
Developing Neos, short for neo spatium - new space, a highly collaborative virtual and augmented reality metaverse.
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 Neos Patreon, subscribing to the "Gunter" level, and linking your Patreon to Discord. Then you will be able to see the #headless-client channel and the password will be pinned in the command after -betapassword. The account will also need to have Neos VR in its library. You can do this by running app_license_request 740250 in steamcmd if you have never installed Neos VR on this Steam account.
For more information related to configuration go here: https://wiki.neos.com/Headless_Client/Server
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, but most communcation is done via NAT hole punch using LiteNetLib (LNL).