Nazi-strict front squats!
Hold the bar on your collarbones/shoulders only.
NO HANDS!

If you lean forward the bar will fall on the floor so keep your back straight and upward!

TIP
Grip your right thumb with your left hand fingers and keep lifting that thumb up through the excercise - that helps to maintain the position. Focus on pushing the floor away from you with your feet and lifting the thumb with your hand :)