Into the Instant Pot, place onion, garlic, chicken broth, key lime juice, and chicken. Sprinkle taco mix on top of chicken.
Place the lid onto the instant pot. Turn the valve to the sealed position. Press "Poultry" Button and cook for 18 minutes if frozen and 15 minutes for thawed chicken. Once the instant pot stops counting, allow to naturally depressurize for 5 minutes, then release the pressure by opening the valve.
Once done, release pressure and shred chicken with two forks. Replace chicken back to Instant Pot and serve.