How to assign a shortcut key to Control-Command-B?

I notice that I cannot assign the key combination Control-Command-B as a shortcut key for any application like Emacs, Spectacle. When setting this key combination into the Preferences of Spectacle, what is recorded is just Control-Command, instead of Control-Command-B.

Does anyone know what is wrong with this key combination?

The reason that I need to use this key is to assign it to the function backward-sexp in Emacs that move backward an s-expression. I’m using macOS Catalina in my Macbook Pro 15” 2019.

Thank you in advance for taking a look at my question.

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Update 1: I forgot to mention that I use Karabiner-Elements to switch Function and Control. So, the physical keys that I want to ask is actually Function-Command-B.