Turn on Instant Pot to SAUTE mode on medium heat. Once the monitor displays hot, add oil to the inner pot and allow it to heat up slightly. Add chicken pieces and allow it to brown for 3-4 minutes, turning them over once they have lightly seared. Lightly sprinkle with salt.
Add onion, celery, mushroom and sprinkle some salt. Stir and lightly sauté for about a minute. Add star anise, coriander seeds and peppercorn. Mix to combine. Continue to sauté for 3-4 minutes or until the mushrooms have slightly browned. Add water, stir and close the lid.
Press CANCEL and choose MANUAL on HIGH pressure. Set the timer to 30 minutes and turn the pressure valve to Sealing. Once the timer beeps, allow it to naturally pressure release for 15 minutes then turn the pressure release valve to Venting and allow the remaining pressure to release. Strain the stock (reserving the chicken) and keep the stock aside for later.
Separate the chicken pieces. Using two forks, shred the chicken and keep aside. Remove the bones and discard.