If you incurred the loss in 2016, and your return is accepted, you would need to amend your return to claim it. You cannot claim a loss that occurred in 2016 on a 2017 return. It is true that you may end up with a carryover amount but the carryover has to stem from items in the return in prior years.
If the return s rejected, you can just add this transaction and refile.
I'll include amend instructions below but it's best of you don't actually mail the amended return until you have received the original refund or the original tax due payment has cleared. Amended returns can only be filed by mail and it takes te IRS 6-12 weeks to process them.
https://ttlc.intuit.com/replies/5567856
