Actually you have to pay the tax due by April 15. The extension only gives you until Oct 15 to file the return. The IRS will mail you a bill for any interest and penalties based on exactly when they get your tax due payment.
You can enter your bank account info for the IRS to pull it out of your bank. Or pick to mail a check and Turbo Tax will print a 1040V voucher to mail in with a check. Or say you will mail a check then pay online at
Https://www.irs.gov/payments
How to pay a federal tax due
https://ttlc.intuit.com/community/tax-payments/help/how-can-i-pay-my-federal-taxes/00/26212
