Its a requirement when hosting a server for it to be password protected.

You can either search in the server finder for "pass, or password" and the name will have the password in the server name.

OR (this is better) google valheim servers. There are multiplayer servers you can join, but you must apply thru discord to get into most.

ALSO use a different character than you SP one, someone may kill you and you dont wanna lose your progress