The $1,800 passed through the insurance is not deductible on your tax return. It was never included in the amount in box 1 of your W-2 because it was part of the health insurance premium that was deducted from your pay, so it has already been excluded from your taxable income.
As you proceed through the HSA section of TurboTax, TurboTax will ask you if your employer told you about any other contributions to your HSA. Answer Yes, then enter $1,800 in the box labeled "Employer and payroll contributions not included on your W-2." This will allow TurboTax to include the $1,800 on Form 8889 in determining whether or not you have exceeded the maximum permissible HSA contribution for 2020.
