Make function keys work on putty's ssh session

When you ssh to a Linux machine using putty and running tmux or byobu, by default, you can not use function keys to switch between sessions (F3, F4) or create a new session (F2). By setting putty as below, you will "fix" that:

1. Go to Putty settings, Terminal section, Keyboard, The Function keys and keypad, select Xterm R6

2. Apply and close.