Sounds like you need to complete Part II of Form 6252. It's buried in one of the Schedule D screens. Here's an article on the subject: https://proconnect.intuit.com/community/individual/help/entering-a-prior-or-current-year-installment...

You'll need some information from the 2018 Form 6252. Also, I assume the payment didn't include any interest but if it did, the interest portion of the payment goes on Schedule B.