It seems to require a connection to Steam in order to play this game. As a user I don't know how to set that up unfortunately.
Viewing post in Rat Stew pt. Two: Future Brew jam comments
Shouldn't need to do anything too wild to get it to point where you can host a lobby and start the game. That said, the game is still in a pretty janky state right now so it might fail for any number of reasons. Thanks for trying it out, though! I'm still working to get through some of the NetCode problems and will hopefully have a solution before the showcase.