Log back into TurboTax and proceed to the Deductions & Credits section.
Once you see the screen titled "Your 2024 Deductions & Credit," scroll down to the section titled Estimates and Other Taxes Paid.
In this section, select start/update to the right of Other Income Taxes.
You will then be taken to a screen titled "Other income taxes." On this screen, select start to the right of Payment with 2024 federal extension and enter your actual extension payment as prompted.