Anyways, so I've found the answer :
In the instructions for airdash, it says to double-tap and release a movement key while in the air... But that wouldn't work for me.
Well, double-tapping movement keys can be replaced with the Shift key. So, while in the air and gliding, tapping Shift triggers the airdash ability.
If you press A (left) and shift, you airdash to the left. And so on.