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Easiest way is to replace it with a custom action that simulates a keyboard key being pressed.
Easy to do
Select the button you want to map (that isn't working) and select "Custom actions" from the dropdown, add a new action, give it a relevant name, click in the "keystroke" field and press the keyboard key you want (in this case it was backspace).
Easier to show you...
I have done this for all the navigation keys as well.