I cannot find where to enter the check I sent when I filed my extension. Where is that located?

Look at line 7 of form 4868. 

To enter the payment you made with your extension in your tax return, type "estimated tax payments" (without the quotes) in the Search box, then click the link that says "Jump to estimated tax payments." That takes you to the screen that says "Estimates and Other Income Taxes Paid." Scroll down a little to the second section, "Payments with Extension." Click the Start, Update, or Revisit button for payment with 2021 federal, state, or local extension, as appropriate.

Stumbled across the location  - wasn't exactly intuitive! But thanks for the replies. Problem solved.