@VolvoGirlSorry for the late reply. I have Bad News and Sort of Good News.
The Bad News: box I found was on the 1040/1040SR Wks, way at the end. It populated the 1040 Schedule 3, line 13z correctly. The override passed all of TurboTax's internal reviews. But the return was rejected "by the IRS" when e-filed. TurboTax said the data in f1040 Schedule 3, line 13z, was invalid or incomplete.
After two phone calls with TurboTax support representatives, the second one consulting with 2d Tier Assistance, and e-filing a second time and being rejected a second time, they instructed me on how to include the payment, and e-filing a third time ...
The Sort of Good News: It was not rejected a third time for e-filing.
The instructions I got from the second representative, in consultation with 2d Tier Assistance, was to add the amount of the additional payment to the amount shown as paid with my extension to file. I told them I thought that would get IRS attention -- providing a different amount than what was paid with the extension form -- but that I would follow their instruction and it is they who will deal with the IRS on my behalf if they're wrong.
My gut says the IRS is going to have a problem with this. I hope not. I will try to remember to update this post if they do.
