Do you mean your w-2? - There is no box 12 on Form 1099-SA. For your 1099-SA, TurboTax will ask if you used all of the distribution on medical expenses. If you answer Yes, the amount is not taxable.
You don't have to enter anything in the additional contributions box unless you make contributions to your HSA outside of your employer. The contributions you (and your employer) made are already included from your W-2, so don't duplicate that amount.