Turn on the Saute mode on your Instant Pot. As soon as the pot is fully heated (to make sure the chicken doesn’t stick to the pot), add the cooking oil. Pan sear the chicken thighs until they turn slightly brown on the surface. (If they stick to the pot too much, you may use a non-stick skillet for pan-searing or skip this step.)
Add the soy sauce, mirin and sugar. Cover the pot and select Manual and set to High pressure for 6 minutes.