The previous selected best answer was hidden and i don't know why. I haven't done my 2024 taxes yet but at least last year I reported on schedule 3 line 13z. Do you own diligence though. Don't blindly trust an internet stranger
Original answer:
The IRS instructions for Schedule 3 line 13z say the following.
"Use line 13z if you made a tax payment that doesn't belong on any other line. Enter "Tax" and the amount of the payment."
You can make the entry in forms mode on the "Form 1040 or Form 1040-SR Worksheet." Open the worksheet and scroll way down, almost to the end of the worksheet, to "Schedule 3 - Additional Credits and Payments." Continue scrolling down to "Part II, Other Payments and Refundable Credits." Look for the "Other Credits and Payments Smart Worksheet," which is a box between lines 12 and 13. Enter the amount of your payment on the last line of that Smart Worksheet, line I. The payment will then appear on line 13z with the description "Tax," which is what the IRS instructions say to do.
