The title says it all. Note that I'm using a non-US layout, Norwegian, so I can't create the Ctrl+] combination. I've found the trick before, but can't find it.
Type Ctrl+] to enter the telnet menu, then enter
quit. For more commands, see
Edit: Haven't noticed that you can't type Ctrl+], but I would be surprised if there isn't a way to type that with every keyboard. But you can change the escape character with the commandline option