How do you add infuse weapon to your skill bar?

I can't click and drag it to the bar. Using a crossbow (non-melee projectile) and it won't work. I put on a spear and still I can't drag it to the bar.

thanks in advance!

It is a trigger skill, which means when you attack, they trigger that skill. When the spear throw makes contact with the enemy, the infusion weapon explodes. This only happened when you consume a charge to gain an infused weapon charge. Did you see a new buff with a number whenever you consume a charge? That is how many infuse weapon explosions you have. They are not an active skill so u not active it to put it in the active bar