subj

There'll be an underslung grenade button on the skillbar.

1. Have a weapon with grenade launcher euipped.
2. Load a grenade into the launcher. Either in the inventory (if you have different kinds of grenades in your squad inventory you can select the kind of greande there like you can select ammo types for normal guns) or with the reload button (a second reload button for the grenade launcher will appear but you can not select the kind of grenade)
3. Fire the grenade launcher with the skill button like you would use any other attack or grenade.