Connect as admin

Edit config.cfg from the game panel > config files
Set your steamID64 in the first admin field
Set an admin password of your choice
Save the config.cfg file, and restart  the server
Once the server is started, connect to it and open your in-game console with F9.
Type admin YOUR_PASSWORD and then press the Enter key
You should receive a confirmation message that you were granted/logged in as admin

Once connected as admin you can add more admins with the following command:
player admin set SteamID64
or
player admin set player_name