With this plugin, the join button actually lets you join a session by getting the steam code of a lobby in your clipboard, then joining it immediately.

To use it, head to the R.E.P.O Discord and copy and code that was given in the #REPO_find_group channel, then you head over to REPO and click join! Boom, you're in.
REPOJoinWithCode.dll in REPO/BepInEx/pluginsMe, and me only. No dependencies (except BepInEx, of course)