Trim boneless chicken breasts and cut into equal sized bite size pieces. Put in a bowl with teriyaki sauce (and onion if you want) and stir so every piece is coated well. Let sit at least 5 minutes to up to an hour if you'd like it to really soak in.
Pour into air fryer basket and shake so they are in one layer overlapping the least amount.
Close and cook for a total of 8 minutes shaking the basket after 4 minutes.
At end check to ensure chicken is cooked thru by taking out largest piece and cutting in half. If it is no longer pink inside you're good, if not add 1 more minute at a time until all chicken is cooked properly and safe to eat.