Update with a note: One have to check for conflicting shortkeys already set for other operations. Else it won't work.

For keyboard shortcuts you do not need to work with the "Contextual Menu"

In the "Keyboard Shortcuts" view:

• Leave the "Context" field to "Application Shortcuts" (for simplicity)
• on the right side "Actions" list navigate to Object -> Shape -> Break Shape and select it
• click on the "Add Shortcut" button (first in the bottom of the "Keyboard Shortcuts" view)
• this will show and highlight the new shortcut item without an assigned key
• click into the second column and press the keyboard shortcut you want to use
• the you can confirm the "Keyboard Shortcuts" view

I tested this here just now and it worked