Help creating a shortcut/hotkey on Mojave

What output do you want? Have you tried the Text section of the System/Keyboard/Text preference pane. Instead of Option+Command you can use some letter combination like xrat to add whatever text you want.

If you're looking to execute some function in an application try the System/Keyboard/Shortcuts/App Shortcuts preference pane.

A 3rd party solution to what you want to do would be to get Keyboard Maestro. It will let you do just about anything with any key or letter combination. I use it and have over 100 shortcuts I use to open files/apps, enter predetermined text, select menu options, etc.

Now that OT mentioned it, both VMWare Fusion and Parallels Desktop use modifier keys as a form of escape key if you don't have their respective tools installed in a guest. So I suppose it's possible, somehow.

It's not possible. Those are modifier keys.