Everything says use Form 8949, but TurboTax doesn't let you fill that form in.
When you get to the point that you are starting to enter the investment sale, choose 'Other' as the type of investment because that will indicate that you do not have a Form 1099-B.
A worthless investment may be deducted as a loss in the tax year that it becomes completely worthless. This means that the corporation has filed for bankruptcy, has stopped doing business, and has no assets.
Entering the information for the worthless stock is the same process as entering any other investment sale, except for the following fields:
All other information will come from your records such as the cost basis and date acquired.
See the following TurboTax help article for guidance to enter investment sales:
Where do I enter investment sales?