Add a custom shortcut in Ubuntu 18.04

So basically, I'd like to do what's written in this answer from 2011 but for Ubuntu 18.04.

I'd like to type an Em-dash from anywhere in my Ubuntu. When I do:

1. Settings > Devices > Keyboards
2. then I go all the way down and click + to add a custom shortcut
3. I name it for instance Em dash
4. enter --. for the command and set the shortcut to Ctrl + Shift + 6

The shortcut doesn't seem to work in anyapp (Atom, Firefox, Terminal etc.). I tried other shortcuts like AltGr + Shift + 6 and I also restarted my computer to see if it'd work. It doesn't. Am I missing something?