You’re correct about the inclusion of the amount on your Schedule A.
According to Premium Tax Credit in the IRS’ Publication 502 (2021), Medical and Dental Expenses, “...[A]ny amount of advance payments of the premium tax credit that you did have to pay back can be included in medical expenses.”
This amount would be included on line 1 of Schedule A, though, as Medical and dental expenses.
Line 2 of Schedule A refers to the Adjusted Gross Income amount from Form 1040. There’s not a line 2b.
