Spray 6 or 8 quart instant pot liner with nonstick cooking spray.
Put water in the bottom of the pot, then the pasta. Pour milk over that.
On top of the pasta, add the tomato sauce, garlic powder, salt and pepper. Try to keep the tomato sauce on top of the pasta if possible. This will help to avoid the burn warning while it cooks.
Cook on high pressure for 1 minute, then do a quick release.*
Stir in cheese until melted then serve.
Notes
*Place a damp rag over the vent when releasing pressure to prevent splatters.Put the water and the pasta in the bottom of the pot and the tomato sauce on top of that. If the tomato sauce is under the pasta, you are more likely to get a burn warning.If you do get a burn warning, don't worry too much. The cook time is so short, it is likely done anyway. If the pasta needs a little more time, just put it on slow cook and keep an eye on it.