I have a 1099-B short-term stock loss for tax year 2016, but missed reporting and hope to apply now. How do I used form 8949 schedule D with Turbo Tax?

You will have to file a amended 2016 tax return to add the Form 1099-B and the stock sale for a loss.

See this TurboTax support FAQ for amending a 2016 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/26768