If you do not have a 1099-B, you should select that you will mail in the 1099-B. TurboTax will then generate Form 8453 which is used to send the additional support.
Instead of the 1099-B, send in the documentation you do have to support the amounts reported on your tax return instead. Make sure to mark the box to the left of Form 8949, Sales and Other Dispositions of Capital Assets (or a statement with the same information).
How do I mail Form 8453?
@dkempson
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
