since the payback occurred in 2021 that's the year for reporting it if at all. the repayment must be more than $3,000 if less no deduction and no tax credit otherwise,
in 2021 you can take as a deduction on schedule A line 16 denote as IRC 1341 repayment
OR
recompute the taxes in the year you received the payment by excluding the amount repaid.
the decrease in taxes is taken as a credit on schedule 3 line 12d denote as IRC 1341 on the 2021 return
nothing has to be submitted with the 2021 return as proof but the IRS could always ask for documentation. therefore save a complete pdf copy of the return both be before and after
Turbotax will not help with recomputing the 2020 taxes. you'll have to open the return noting what the taxes are and then reduce wages by the amount repaid and see what the revised taxes are. the reduction is the credit to be taken in 2021.
generally, the credit is better but since tax calculations can be quite complicated, you'll have to try both options to see which is better.
