When did you pay it? Was it for a 4th quarter estimated payment or a payment with an extension to file?
To enter the amount you paid with an extension (including state extension) go to
Federal Taxes (or Personal for desktop H&B version)
Deductions and Credits
Then scroll way down to Estimates and Other Taxes Paid
Other Income Taxes - Click the Start or Update button
Next page second section Payments with Extension
Either Visit All or Click Start or Update by the extension you paid
If you prepared an extension in TurboTax, the amount of your payment should have been entered automatically. It would appear on Schedule 3 line 9. If it's not there, type "estimated taxes paid" (without the quotes) in the Search box, then click the link that says "Jump to estimated taxes paid." That takes you to the screen that says "Estimates and Other Income Taxes Paid." You can enter either estimated tax payments or a payment with an extension on that screen. Click the Start or Update button for the appropriate type of payment. You might have to scroll down a bit to see "Payments with Extension." If either type of payment has already been entered, you'll see it when you go to the entry screen.
Estimated tax payments would appear on Form 1040 line 26.