Hi Flowerscat,
Since you want to create a new key combination shortcut on your Mac, we recommend that you try the steps here: Create keyboard shortcuts for apps on Mac - Apple Support
To perform the recommended troubleshooting steps via the link above, complete the steps in the section below.
1. On your Mac, choose Apple menu 
> System Settings, click Keyboard 
in the sidebar (you may need to scroll down), then click Keyboard Shortcuts on the right.
2. Select App Shortcuts on the left, click the Add button 
, click the Application pop-up menu, then choose a specific app or All Applications.
For example, to set a shortcut for a TextEdit command, choose TextEdit. If an app isn’t in the list, choose Other, then locate the app using the Open dialog. Some apps may not allow you to set keyboard shortcuts.
If you want to set a shortcut for a menu command that appears in many apps, choose All Applications.
3. In the Menu Title field, type the menu command for which you want to create a shortcut, exactly as the command appears in the app, including the > character (type ->), ellipses (type three periods without spaces or press Option-; (semi-colon)), or other punctuation.
For example, to set a shortcut for the default ligature command in TextEdit (Format > Font > Ligatures > Use Default), you would type Format->Font->Ligatures->Use Default in the Menu Title field. To set a shortcut for the Export as PDF command (File > Export as PDF…), you would type File->Export as PDF… in the field.
4. Click in the Keyboard Shortcut field, press the key combination that you want to use as the keyboard shortcut, then click Done.
For example, press the Control, Option, and Z keys at the same time. You can use a key only once in a key combination.
We recommend the steps above because they will show you how to create keyboard shortcuts for apps on your Mac.
Please let us know the outcome after giving the above steps a try.
Thanks for using the Apple Support Communities.
Cheers!
