How to make the fn key show mission control

So I wiped my computer to start fresh and can't figure out how to make my fn key show mission control like I had setup prior. Before the reset, all I had to do was push fn (no other buttons) and it would show all my open programs. I went to the shortcut and tried to change it, but when I try to make it only fn, it does nothing. I did fn+left arrow and that worked, but required 2 keystrokes