"YOUR STEAMID" "99:z"
ensuring you keep the quotation marks
(Adding 99:z will give the user full access to your server and all of its commands in game. If adding someone other than yourself as an admin we highly recommend changing this as they will have the highest privileges available on your server. For more information about this setting we highly recommend you visit https://wiki.alliedmods.net/Adding_Admins_(SourceMod)