Wash and soak the dried adzuki beans in a bowl with enough water to cover (at least 8 hours, or overnight).
Drain the water, and place the beans into the Instant Pot liner.
Cover the beans with 4 C water and add in rock sugar.
Place the lid on, set to vent to SEAL and press the BEAN/CHILI function -- 25-28 minutes, HIGH PRESSURE.
Once the beans have cooked, open the lid and use an immersion blender to blend the red bean soup.
Or transfer about half of the beans and some liquid to a blender and blend until smooth. Return the blended mixture to the pot and stir. You can skim off and discard the foam.
Depending on your preference, you can blend the entire soup for a smooth red bean soup, or you can leave some beans unblended for texture. Add additional water if you feel the soup is too thick.