You will need your steamid64. To find it is not very complicated, copy the url (web address) of your steam profile page and go on http://steamidconverter.com/
Then paste your profile address and click "Convert". You can then get your SteamID64.
Method 1
From the game panel > config files edit the file /server/roxservers/cfg/users.cfg
Add the following line to set yourself as "owner":
ownerid STEAMID64 "your name" "reason"
or the following line to add a moderator:
moderatorid STEAMID64 "name" "reason"
obviously replaced by the steamid64 STEAMID64 obtained above.
Save and restart the server.
Method 2
Download Rustadmin RCON Tool
Launch Rust Admin and connect to your server
If you forgot your rcon port it is simply your server port + 2.
Go to the console tab and type the following command in the filed marked ""type a command to execute"":
ownerid <your steamid64>
obviously replace <your steamid64> steamid64 previously obtained.
After adding administrators send the command server.writecfg
This will write the configurations to files that you find in /server/roxservers/cfg/
You will find the users.cfg containing moderators and owners you added.