Hello aosorial:
The solution to your question is a little inconvenient, but also very straightforward.
What you should do is to immediately file an Amended tax return for the 2016 tax year, and then attach a completed Form 8938 to it. You don't need to make any other changes to your original tax return (unless you do for something else). As for a reason for amending, in the description field for Form 1040X you can enter something like "To Add Form 8938 Disclosure, Not Included With Original Return."
Instructions on how to start with amending a tax return in TurboTax 2016 can be found at the following link:
<a rel="nofollow" target="_blank" href="
https://ttlc.intuit.com/questions/3061514-how-do-i-amend-a-2016-return-in-turbotax">https://ttlc.int...>
I've also added a (new) screen-capture image to my original answer for your reference as well (it is a copy of the relevant section of Page 1 of the Form 8938 instructions booklet). Simply click the icon to open.
Thank you.
