How can I turn off my Mac's internal display?

How can I switch off the internal display when having externals connected - WITHOUT closing the screen. Dimming the screen DOES NOT switch it off as windows can still be "shown" there you just don't see it. I have many apps that ask for the finger print and this makes it very distracting when the screen isn't off.

On normal laptops this is a very easy thing to do normally just a hotkey combo.

[Re-Titled By Moderator]