For the contribution to be "post-tax" with respect to your employer, your employer must have included the amount in box 1 of your W-2 (which makes no sense if they are treating your payroll deductions to the HSA as pre-tax).
If their contribution is included in box 1 of your W-2, you'll have to report it as your own personal contribution so that you get the deduction for the contribution to effectively convert it back to pre-tax. If the employer did not include their contribution in box 1, tell TurboTax that your employer told you about other HSA contributions and enter the amount as an amount not reported in box 12 of your W-2. TurboTax will then include it on Form 8889 line 9 along with the code W amount.
