I´m toggling “lock camera to view” quite often, yet I can´t find a way to toggle it with keyboard. Preferences-Keymap come up empty. Given how flexible and customizable Blender is, and how prominent this feature is, I find this quite weird.
Add this mapping in 3D View (Global) (you can of course use your own shortcut):
You can also just right-click the checkbox and use the ‘Assign Shortcut’ from context menu, but that creates the shortcut in the wrong context and that may cause problems.