Press the jump button while you are in the air. So for keyboard and mouse, that is space.

hold jump.
the jetpack has limited height, it's more like a hoverpack than a jetpack.

Equip the jetpack BP
I hold the W and the spacebar, holding just the spacebar alone lifts me up.
Jet pack 5 we can go pretty high .

Wow, that takes me back.
That was the first question I had way back in the beginning.
Fly free....

Essentially Jump then hold Jump Key. So it's a kind of double jump really.

The trick is to throw yourself at the ground...and miss