Make the meatloaf: Mix beef, bread crumbs, onion, apple, egg, garlic powder, salt, and pepper in a large bowl until evenly blended.

Shape the beef mixture into a loaf and place on a large piece of aluminum foil. Fold the foil up and around the edges of the meatloaf, creating a makeshift loaf pan.

Pour 1/2 cup water into a multi-functional pressure cooker (such as Instant Pot) and place the steam rack inside with the handles up. Place the meatloaf on top of the rack. Close and lock the lid; seal the vent. Select high pressure according to manufacturer's instructions; set timer for 25 minutes. Allow 10 to 15 minutes for pressure to build.

Set an oven rack about 6 inches from the heat source and preheat the oven's broiler.

Release cooker pressure carefully using the quick-release method according to the manufacturer's instructions, about 5 minutes. Unlock and remove the lid. Transfer meatloaf, still on the rack, to a baking sheet. Broil meatloaf in the oven until browned, about 5 minutes.

Meanwhile, make the topping: Combine ketchup, mustard, and brown sugar in a small bowl. Brush over meatloaf and broil again until caramelized, 1 to 2 minutes.