If you have already filed the return, you will need to file an amended return, Form 1040 X and attach Form 8938 to the amended return unless the return is rejected.
If the return is rejected, you can then add the form to the return before you resubmit.
It is best to wait to file the amended return until after you receive your original refund.
To amend, log back into TurboTax.com or open your TurboTax software installed on your computer.
Select the option “Amend a return that was filed and accepted.” and add-in the missing information to generate the form.
You can see more detailed instructions on the amendment process here: How to amend a return in TurboTax.