I sent check to IRS when I asked for extension at last April. How I add that amount as a tax payment

When you requested your extension you were supposed to pay your estimated tax due.  Did you enter the payment you made into your tax return?  Go to Federal>Deductions and Credits>Estimates and Other Taxes Paid>Other Income Taxes 

If your extension payment did not automatically show up on 1040 you need to enter it.  So make sure it's not already there before you enter it!

For 2021 Federal Extension payments should be on schedule 3 line 10. And go to 1040 line 31.

You can type estimates paid in the search box at the top of your return and click Find and it will give you a link to Jump To the screen where you enter Extension Payments.

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