- Visit https://steamid.io/
- Find your own profile on steam (for example: https://steamcommunity.com/id/examplesteamprofilename)
- Enter the URL of the profile into the lookup on steamid.io
- Copy your steamID64
- Open the cp.elitegameservers.net controlpanel website, go to configuration files and edit the "Servers\VServer\Server\Commands.dat" file
- On a new line; paste the steamID64 and add the text owner in front of it.
For example: owner 7656119xxxxxxxxx - Save the file and restart the server.
The commands are listed on: http://unturned.wikia.com/wiki/Server_Commands
Note: you can add more players as admin by adding them with the /Admin command. For more details see the above link.