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To become an admin you first need your SteamID64.
You can find out how to get it here: How to find out my SteamIDs .
In Basic Mode you enter the SteamID64 under Server Settings -> Admin Settings -> Admins.
In Advanced Mode you enter the SteamID64 in the ownerswhitelist.txt.
By default, the entry looks like this:
# Jane Doe
7987654321987
# John Doe
7123456789123
Note the following:
When you have made your changes click Save and restart your server.
You can access the admin menu ingame by pressing [ESC].