I don’t think that’s in the rules. Like, at all. The unarmed fighting style allows you to deal damage to a creature grappled by you, the grappler feat allows you to pin a creature you grappled (which is just fucking useless since both of you become restrained), and you can make a shove attack to push a creature prone. But there’s nothing in the basic rules about an unarmed attack that deals damage and knocks the target prone.
The alternatives for flavoring are:
Battle Master fighter, trip attack. Technically it must be a weapon attack, but if you have the unarmed fighting style, a natural weapon, or are a monk multiclass, I’d be inclined to allow it.
Open Hand monk, Open Hand technique. This is probably the best alternative that is 100% RAW.
Of course a more permissive DM (like me) could allow you to make a fairly hard athletics check once you have grappled the orc and have two free hands, then resolve it as a 2d6+STR bludgeoning damage attack.