How/where do I indicate/mark that I filed an extension since I didn't prepare the extension in TurboTax?

[DoninGA]

There is no where on the tax return where you indicate that you filed an extension request.  If you filed an extension and paid taxes with the extension then you need to enter the taxes paid on your tax return.

To enter, change or delete a payment made with an extension request (Federal, State, Local) -

• Click on Federal Taxes (Personal using Home and Business)
• Click on Deductions and Credits
• Click on I'll choose what I work on (if shown)
• Scroll down to Estimates and Other Taxes Paid
• On Income Taxes Paid, click on the start or update button

On the next screen select the type of extension payment made and click on the start or update button

Or enter federal extension payment in the Search box located in the upper right of the program screen. Click on Jump to federal extension payment

The federal extension payment will be shown on Schedule 3 Line 9. The amount from Schedule 3 Part II Line 13 flows to Form 1040 Line 31

[VolvoGirl]

You don't need to show that.    The IRS knows if you filed an extension.  Did you make a payment with the extension?

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 2020 Federal Extension payments should be on schedule 3 line 9.  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

[Mike9241]

you don't. if you successfully filed one, the IRS has a record. if you made a payment that must be entered. 

1. Open or continue your return in TurboTax, if it's not already open.
2. Search for estimated tax payments (use this exact phrase) and then click the Jump to link in the search results.
3. In the Payments with Extension section, select Start or Update next to the payment you submitted with your type of federal, state, or local extension. Make sure the amount shown is correct.