You're asking about 'pinch' harmonics. A keyword search (use the button in the toolbar just above thiis thread) will bring out a lot of info on these.

Basically, hold the pick so that the tip of your thumb is close to the tip of the pick. Then allow the thumb to lightly touch the string just after you pick a note. If the point where your thumb touches the string is at the right distance from the bridge, it will cause the string to vibrate at a harmonic of the fretted note.

Experiment with thumb/pick position, and placement along the string length. It will soon become second nature to you.