Please read the TurboTax FAQ for information on "How do I file an IRS extension (Form 4868) in TurboTax Online?": How do I file an IRS extension (Form 4868) in TurboTax Online?
For more information - "How Do I Get An Extension on Taxes?": https://turbotax.intuit.com/tax-tips/tax-extensions/how-do-i-get-an-extension-on-taxes/L5ThSke8RSo I still need to pay the taxes owe even if I have an extension? What if my tax return is still incomplete after the deadline? My taxes owe would be incorrect if I had to pay before the April deadline.
Pay your best estimate of the tax that you owe with the extension. You'll settle up for any difference when you file your tax return. If you end up owing a significant additional amount you might be charged a penalty for late payment. The only alternative is to pay the maximum that you could possibly owe with the extension, and plan on getting a refund when you file the tax return.