To activate the Nitro Boost Controls press "Right Ctrl + Numpad Enter"
You will see a notification at the bottom of the control help gui when Nitro is activated.

-Hold 'Spacebar' for a Nitro Boost.
-Hold 'Left Shift + Spacebar' for a Reverse Nitro Boost.
-The "delete" key can be used to stop the vehicle instantly, even in the air. (When Nitro Boost is available)