The box is blank on the 1099-B because the brokerage or financial institution does not have the information. But it's a required entry on your tax return. You have not said which box is blank on your 1099-B. In order to help you with what to put in the box, we need to know which box it is. If the transaction is anything other than a straightforward sale of a security that you owned, also describe the nature of the transaction.
