I had a refund on my taxes in 2020,but chose to apply to my 2021 estimated taxes, where do i enter that credit on my 2021 taxes?

Look on line 36 of the 1040 wks. You'll find this in the list of forms under the "View" "Forms" selection from the dropdown menu. It should be the first form. On line 36 you can indicate the amount of the refund you want applied to 2022 taxes.

If you used TurboTax (TT) for 2020, that should happen automatically.  

To verify, In TurboTax (TT), enter at:

Federal Taxes Tab (Personal for H&B version)

Deductions & Credits

-Scroll down to:

--Estimates & other taxes paid

   -Other Income taxes

    On the next screen, scroll down to:

     -2020 Refund applied to 2021 Taxes

        -2020 Refund applied to 2021 Federal Taxes

On the next page, the box should be prepopulated with last year's refund.  If not, change it to the correct amount.