You will be able to amend your return to add the document (once it is filed and accepted, the amendment process is how you add forgotten forms).
TurboTax will walk you through the amendment process and I have included a link below that will help you get started. However, you are going to want to wait until you have actually received your first refund before you start the amendment (this is the advice that the IRS gives). This is because if there are any other issues in your return or forms that come up while the IRS is processing your return, you do not want to have to amend a second time. Amending an already amended return can be a cumbersome process.
Amendments will need to be paper filed, so you will have to physically print and mail the amendment. Amendments take awhile to process (up to 16 weeks) and both of your returns will be processed separately. So, if you are expecting a refund from your original return and you owe on your amendment - keep in mind you will have to send in a separate payment for your amendment, but you will get your original refund within the normal timeframe. If you wind up getting more money back from your amendment, then that will come in the form of a paper check.
