Depends on your OS. Mac is the easiest, it's just ---, Linux depends on your distro, if it uses KDE, it's <right-win>--- —. Windows is a little awkward, I think you need <right win>+the code point.
edit: another comment gave a mac shortcut – — - <--- one of these might be it
Just --- only works when you have the text replacement thing on (the same thing that turns (C) into ©).