File an amended 2017 return. Since this provision expired on December 31 of 2017, you need to file an amended 2017 return to retroactively claim the credit. If this results in carry-forward credit (credit you could not use in tax year 2017), then you will be able to claim the carry-forward portion of the credit to your 2018 return. But to claim the original credit, you'll need to amend your 2017 return.
This FAQ will assist you to do this: https://ttlc.intuit.com/replies/4643391
File an amended 2017 return. Since this provision expired on December 31 of 2017, you need to file an amended 2017 return to retroactively claim the credit. If this results in carry-forward credit (credit you could not use in tax year 2017), then you will be able to claim the carry-forward portion of the credit to your 2018 return. But to claim the original credit, you'll need to amend your 2017 return.
This FAQ will assist you to do this: https://ttlc.intuit.com/replies/4643391