You can use the Steam dedicated server software even if you have only the non-Steam game. You don't need Steam login to download or start the dedicated server. Like is told above, just add the -crossplay command to the starting bat or shell script, which actually should be there already by default.

Here's a handy guide how to setup a dedicated Valheim server: https://valheim.fandom.com/wiki/Valheim_Dedicated_Server