If you have filed your tax return and then discover that you have Form 8962 to file you will have to wait until the IRS either accepts or rejects your tax return.
If the return is rejected you will correct the error that caused the rejection, add Form 8962 and refile your return.
If the return was accepted by the IRS then you will have to file an amended tax return using Form 1040X to add Form 8962 to your tax return
This link to the TurboTax Help page How to amend (change or correct) a return you already filed provides information on how to amend your tax return.
**Mark the post that answers your question by clicking on "Mark as Best Answer"