Go to the webpage: https://www.irs.gov/forms-pubs/extension-of-time-to-file-your-tax-return
Go to paragraph: E-file your Extension Form for Free and select "free file" link. You will need to enter estimate and pay your estimated liability.
Or on the same page in the next paragraph, you may simply select Make a Payment. On the form select reason as Extension and you will automatically file the extension with the payment.
Thanks
yvette