How can I attach a 1099-B to my return as proof of cost basis?

It depends.  There is no way to attach a document to your return to be filed electronically. You would need to mail in the 1099-B along with a 8453 form. Taxpayers normally need this when they summarize their stock transactions in Turbo Tax

 

If Turbo Tax identifies your need to mail this statement in, an 8453 should be included in your tax forms. You will print this form out and send in along with the 1099-B form to:

 

Internal Revenue Service

Attn: Shipping and Receiving, 0254 Receipt and Control Branch

Austin, TX 73344-0254 

 

Turbo Tax includes the complete instructions for this also in the software.

Keep your records of cost basis for at least three years.

IRS will contact you if they see any issue with the basis amount you reported.

You don't have to attach or mail anything.