I've been searching for a macOS keyboard shortcut to insert a checkbox before an item. The only ones I found did not work for me. I finally had to create my own Text Replacement to do this.
Open System Settings. Search for Text Replacements. Click the button Text Replacements. Click the + sign. In the Replace field, type -[]. In the With field, type - [ ]. Then click Add. Then click Done.
Now, to add a checkbox in the markdown panel, type -[] (note the trailing space) and you'll get - [ ] . I could find no better way than this unfortunately. But it is faster than typing - [ ] outright.
No, and cmd-shift-9 does not work either.
I use Karabiner-Elements only to get Quit application by pressing command-q twice so I can avoid quitting with CMD-Q when I mistype CMD-W.
Some of those other tools look interesting but will require some time to set up and learn.
I prefer to use native solutions whenever possible instead of apps or browser extensions.
For example, the Eject key remapping example is much more easily done in System Settings->Keyboard shortcuts->App Shortcuts->All Applications->Sleep->ALT-CMD-F12 where F12 is in the same key position as EJECT on an Apple keyboard. This is for my much better Keychron keyboard.
Under Linux and Windows It only works in the RTE editor and not in the markdown editor...
If the checkbox is not a part of a list you should be able to add it like a character (like in a text editor). You need to find its code number.
I think you can also use the character palette (control+command+Spacebar), search for ballot, and then double-click the character to insert â into your Pages document.
If it's OK for you, use then a keyboard customizer (see previous post above) in order to link the shortcut of your choice to this symbol.
I think in Markdown it's the only solution...
Why not to click on the 
button in the menu? Sounds quicker to me.
If you want just to add the actual symbol - â, you can use replace text in settings like this:
or use any other abbreviation instead "chk" you like.
