You can use SteamCMD (https://steamcdn-a.akamaihd.net/client/installer/steamcmd.zip) to download and host a dedicated server without having to own the game.

The quick and dirty steps

1) Download and extract SteamCMD
2) Launch steamcmd
3) type "login anonymous" (without quotes)
4) type "app_update 896660" (without quotes)
5) launch the server from steamcmd\steamapps\common\Valheim dedicated server

If the game patches and server needs to be updated shut down the server and repeat steps 2-4