Place chicken breasts in bottom of Instant Pot. Add in chicken broth.
Place lid on top of Instant Pot, move valve to SEALING, and press PRESSURE COOK button (or MANUAL button). Set timer for 18 minutes.
When timer is finished, move valve to VENTING and do a quick release of the pressure.
Remove chicken and shred using 2 forks. Place shredded chicken in a large mixing bowl and toss with BBQ sauce.
Cover bowl with plastic wrap and refrigerate until chicken is cooled.
Add mayonnaise, onion, celery, salt and pepper flakes to chopped chicken; mix together. Serve on buns or rolls with tomato and lettuce.
