Adding addmins to your server allows you or anyone you set as an admin to manage your server using in-game commands such as /admin.
1. Click on Factorio in the sidebar to open the console for the server.
2. Make sure the server is currently running then click in the textbox of the console where it says
Enter your command or message here and press enter to send
/promote <username> replace
<username> with your in-game user name or the username of the player you wish to set as an admin then press enter
4. If you are logged in on the server you should hear a chime and if you type
/admin it will open the in-game admin interface.