If you filed your tax return already with an accountant for 2018, then in order to file the information on the 1099-B an amended return would need to be filed.
It is possible to create and file an amended return for this by using TurboTax, but you would first have to re-create the entire original tax return as was filed by the accountant and then create the amended return. Instead, your best course of action may be to discuss this with the accountant so that they can do the amended return (your original return would already be in their system).
Do not stress over reporting the additional loss on your return right away. You have up to three years to file an amended return.
