How to connect to a custom server:
Paste the server IP in the text box under the name text box.
Or, press the F12 key, then look for the console tab, and finally type setServer("serverIP:port") and press enter.
If you cannot connect to the server, check if you have some anti-virus or firewall blocking the connection, make sure you selected a server to connect to, and/or make sure the server you're connecting to isn't down.