If you already know and understand SteamIDs then you will fly through this! if not then no worries I will cover how to get these and where to place them.
Finding SteamIDs
You will need to find out your or the players Steam64ID which you can do so easily here on this link: https://steamid.io/
Simply enter their name in the lookup box and grab the id on the line: steamID64 e.g. 76561197967057037
If you do not know what to enter, you can also find out the ID within the Web Console or Log Viewer in the control panel.
When the player connects that you want to find out their ID it will show up like this:
Got connection SteamID 76561197967057037
You can also find out in-game! If you hit F2 in the 'Players' section you will see the players name and ID.
If your server is running crossplay
Anyone joining from Xbox or Game Pass has no Steam64ID at all, so there is nothing to look up for them on steamid.io. Iron Gate gives every player a Platform User ID instead, which is the platform name, an underscore, then their ID, and it is case sensitive. You will see it in the Web Console or Log Viewer when they connect, and they can read it off the F2 panel themselves.
It goes into the same file on its own line, exactly like a Steam64ID does. Steam players are unaffected by any of this, a plain Steam64ID on its own line still works fine.
Adding it to your adminlist.txt file
In the control panel, open Configuration Files in the sidebar and choose the one described as 'Configure admins'. Your bannedlist.txt and permittedlist.txt are sat in there too. If you would rather edit the file directly it lives at Valheim\adminlist.txt in the File Manager.
Simply put the ID on a new line e.g like the image below!
// List admin players ID ONE per line
76561197967057037
Click 'Save & Exit' and restart your server! You or the player will now be an admin!
Using the new found administrator powers!
To use the in game admin menu, you need to ensure you have configured the launch option in Steam!
In your steam library, right click on Valheim and select Properties
In the general tab, you will see launch options, enter -console
To find the admin commands click on this guide here: https://low.ms/knowledgebase/valheim-admin-commands