Combine ground beef, bread crumbs, milk, Cheddar cheese, egg, and dry onion soup mix in a bowl; mix until well combined. Transfer mixture to a cutting board and shape into a loaf.

Place a trivet into a multi-functional pressure cooker (such as Instant Pot®). Pour 1/2 cup water into the pot and set loaf on top of the trivet. Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 15 minutes. Allow 10 to 15 minutes for pressure to build.

Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid.

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

Carefully transfer the trivet to a rimmed baking sheet. Brush meatloaf with barbecue sauce. Broil for 3 to 4 minutes. Allow to rest for 5 minutes before slicing.