Apologies for cultural confusion :D

Okay so, depending on your server host they might automate some parts of this, if not you'll probably want to install a modded server on local hardware first, make sure that's working then upload the files.

Step one of that is installing SteamCmd https://developer.valvesoftware.com/wiki/SteamCMD#Downloading_SteamCMD

Then you make a folder for the server, copy the SteamCMD folder into it and run the update command from there:
steamcmd.exe +login anonymous +force_install_dir ..\Application +app_update 896660 validate

That should leave you with a current valheim_server.exe in <Server Path>\Application\

Download the usual https://valheim.thunderstore.io/package/denikson/BepInExPack_Valheim/ and unpack that into <Server Path>\Application\BepInEx\

Copy mod .dll files into <Server Path>\Application\BepInEx\plugins\

Test run server from <Server Path>\Application\ with command:
valheim_server -nographics -batchmode -name "Server Name" -port 2456 -world "Save File" -password "451" -public 0