Enter the contributions you made outside of what was deducted from your payslip. This amount should be $7,200-3,712.50 or $3,487.50.
You have already received the tax benefit of the remaining contributions you made from your paychecks. That is because this income was not added to your taxable income for federal income tax, or social security or Medicare taxes. You will now get to deduct your remaining contribution, the $3,487.50, from your taxable income. That means that contribution will also reduce your taxable federal income.
You do not need a corrected W-2. Your employer has reported it correctly if the remaining $3,487.50 was made out of your own pocket. Code W on your W-2 includes both employer contributions and contributions you have made of pre-tax income.
Once you report it this way in TurboTax, assuming you had family HDHP coverage for the entire year and had no over-contributions in prior years, you will not be assessed an over-contribution penalty in 2021.
