Hi Tones,
Overview: I had to create two tiddlers, then go into Control Panel -> Keyboard Shortcuts to set the key combination for the shortcut.
I called my shortcut "canscut".
First I made a tiddler with an arbitrary title, whose text field contains the action I want the shortcut to invoke; in this case:
<$action-sendmessage $message="tm-modal" $param="HelloThere" />
I added a field with name key and value ((canscut)), and tagged it $:/tags/KeyboardShortcut. That takes care of setting up the actions and the reference key so the shortcut can be referred to in macros without being opinionated on what its specific key combo should be.
Then I created the tiddler $:/config/ShortcutInfo/canscut -- I didn't have to put anything into it; that's enough to add a row into the appropriate part of Control Panel -> Keyboard Shortcuts, so that the key combo can be set interactively. Once I used that interface to set the key combination, the tiddler $:/config/shortcuts/canscut was generated with shift-cmd-G in the text field.
And voila, now if I hit shift-cmd-G, I get the HelloThere tiddler in a modal.
Hope that helps,
Chris