TurboTax will generate Form 8453 when you go through the final filing steps and select e-filing. It waits until then because Form 8453 is not used if you file by mail. If you file by mail, you attach a copy of the 1099-B to your paper tax return, after all the IRS forms.
Whether you e-file or file by mail, send the IRS a copy of the 1099-B, not the original.
