How to become an admin on your L4D2 server
- Firstly, navigate to your File Manager.
 

- 
Then go to
left4dead2 => addons => sourcemod => configs => admins.cfg. - 
Scroll down to the end of the script until you find:
Admins { } - 
Now you need to input your info in this format:
 
"PlayerNickname" {
"auth" "steam"
"identity" "insert_players_steamID"
"flags"  "z"
"immunity"  "99"
}
In the end, it should look something like this:
Admins
{
  "PlayerNickname" 
  { 
   "auth"  "steam" 
   "identity"  "insert_players_steamID" 
    "flags"  "z" 
    "immunity"  "99" 
  }
}

- Once done, click on Save Content and Start/Restart your server.