What is the keyboard shortcut to assign a keyboard shortcut to a menu command?
Word for Windows: Ctrl+Alt+Numeric Keypad "+"
My preference, though, is to assign them through the Customize the Keyboard button on the Customize the Ribbon dialog. Here is my article on Assigning Keyboard Shortcuts in Word 2007-2019.
See my other article on creating a list of keyboard shortcuts (I recommend to pdf).