Note: You will need your SteamID64, which you can obtain, among other places, here: https://steamid.io
Simply enter the following values in the Admin Settings section under Admin 1 to 5 on the web interface.
Save the changes and restart the server.
Simply enter the following values in the user.cfg file under Admin 1 to 5 on the web interface.ownerid SteamID "SteamPlayerName" "reason"
Example: ownerid 1234567890 "SteamPlayer" "Owner"
Save the changes and restart the server.
Note: You can find a list of commands you can use HERE.