If you want to e-file your 2018 tax return using the correct basis but without TurboTax prompting you to provide an explanation, you would have to clear your 2018 tax return, start over and not transfer in your 2017 tax file. This would require you to manually enter any other information from your 2017 tax file that needs to transfer, perhaps not a good solution. Otherwise you could prepare the explanation but print and mail your 2018 tax return, omitting the explanation page (or portion of page). I think explanation should be included, though.
