You can elect to take the HCTC only if (a) you were an eligible TAA, ATAA, or RTAA recipient or PBGC payee in 2017. But please read the Special Instructions on page 1 of the Instructions for Form 8885 - in most cases, you have to already be receiving benefits to qualify for this special treatment. https://www.irs.gov/pub/irs-pdf/i8885.pdf
Also read the 2017 IRS page (see "Proof of Eligibility") where you have to have received "[a]n official letter reflecting that you were an eligible individual for the months claimed in 2017". See https://www.irs.gov/credits-deductions/individuals/claiming-2017-hctc
If you pass these eligibility tests, then, yes, you would amend your 2017 return.
