- Shut down the server
- Click on Configuration Files in the main menu
- For the adminlist.txt file, click on the text editor
- Enter your SteamID64 (one ID per line) https://www.steamidfinder.com/
- Click Save and exit
- Start the server
You now have your rights set, but to use the commands you need to enable the console in the game.
You can do this in the General/Console Enabled settings and check

The console can then be opened using the ` button above the "TAB"
To view the command list, type "list"
