Start by looking at your own bank and credit card records from April 2018. If you entered the information onto your tax return when you made the payment with your extension then it will show up on line 70 of your Form 1040.
Go to Federal>Deductions and Credits>Estimates and Other Taxes Paid>Other Income Taxes to enter the payment if you have not already entered it.
The very last day to e-file a 2017 return is October 15, 2018. After that, e-filing for 2017 will close down permanently, and 2017 returns will have to be printed, signed and mailed. It is recommended that you get it e-filed before that deadline if you can, “just in case” of last minute problems.