For 15 minutes let the rice soak in:
Turn the instant pot on to sauté mode. Cook the rice and water until the mixture steams. (This prevents the milk from burning).
Close the lid, seal the valve, and cook on either high pressure for 20 minutes or use porridge mode. Use natural release to avoid milk splutter.
The instant pot's normal "keep warm" feature works for hours. (Do not try using sauté mode to keep the rice pudding warm, or it will burn onto the bottom of the pot.)
Serve warm or chilled.