If the taxes were withheld from your distribution, they would be shown on the 1099-R and you enter them when you enter the 1099-R. To enter the 1099-R, type 1099-R in the search box and click on jump to 1099-R in the drop down box.
If you paid the IRS separately from the distribution and the taxes were not withheld from the distribution, then you would enter them as estimated taxes paid. Type estimated tax payments in the search box and click on jump to estimated tax payments in the drop down box.
