If you are e-filing a tax return you do not physically sign and date the tax return.

If you are going to print your tax return for mailing then see this TurboTax support FAQ for printing a tax return for mailing when using the online editions - https://ttlc.intuit.com/questions/1944348-how-do-i-print-and-mail-my-return-in-turbotax-online

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 Other Income Taxes, 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

The federal extension payment will be shown on Form 1040 Line 70

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