"How do I get the return to use the signed attachment?"
TurboTax Business does not save the signed 8453-S attachment in its list of forms that you prepared.
Rather, the program uses the file that you uploaded from your computer (the one where you printed the 8453-S, signed and then scanned the signed form and saved that file to a location on your computer).
Again, TurboTax uses the correct form and, if you need a copy, you can simply print it out from the PDF file you generated and save it with the rest of your printed return.
