Add olive oil to Instant Pot and press SAUTE button.When screen reads HOT add onions and cook 2 minutes.Press CANCEL.
Place chicken on top of onions and season with salt, pepper, chili powder and Tajin. (add cayenne if using)Pour chicken broth around chicken.
Close lid and turn pressure valve to SEALING.Choose PRESSURE COOK mode and adjust timer to 7 minutes.When timer is done, allow timer to count up 10 minutes for Natural Pressure Release.Press CANCEL.
Carefully turn pressure valve on lid to VENTING to release remaining pressure.
Open lid, transfer chicken pieces to a plate, and using 2 forks shred the meat.Return chicken to Instant Pot.
Add Fajita seasoning from packet to the Instant Pot and give everything a good stir.
Close lid and turn pressure valve to SEALING.Choose PRESSURE COOK and adjust timer to 3 minutes.
When done, carefully turn pressure valve on lid to VENTING to release all pressure.Open lid and give everything a good stir again. If there is too much liquid, press SAUTE button and cook for a few minutes to reduce.
At this point the chicken filling is ready. If using later, close lid and leave until needed.If using immediately, transfer chicken to a serving bowl and serve along with wraps and all the toppings and enjoy.