Pour chicken stock into the Instant Pot insert and place the frozen chicken inside the liquid (or put a trivet inside and add the chicken on top of the trivet), making sure they are not stuck together.
Cover your Instant Pot and set the valve to the sealing position.
Select the manual pressure / pressure cook button, select high pressure, and set the timer for 10 minutes - 12 minutes (depending on the size).
When done, allow for a natural pressure release of 5 minutes (10 minutes for thicker chicken breasts), then perform a quick pressure release and open the lid.
Make sure to check the temperature of the chicken using a Meat thermometer.
Notes
To season an Instant Pot frozen chicken breast, you can either season the chicken breasts before adding them to your freezer (my preferred method), or you can cook without the spices and season after cooking.
It is best to separate the frozen chicken breasts before cooking them to avoid ending up with undercooked chicken. The easiest way I find is to 1) freeze them individually or 2) if frozen in a batch, let them sit in a bowl of cold water for a little while until you can easily separate.
If you prefer, you can cook the frozen chicken on a trivet inside the Instant Pot.