Opus 17 is correct; you must tell the HSA custodian that the contribution in 2019 is for 2018 because, by default, the custodian will use the current year (2019). If there is doubt in your mind, call them now.
Having said that, you enter a 2019 contribution for 2018 as a "personal" contribution on your 2018 return.
Open TurboTax and go to your tax return. Do a Search for hsa (lower case) and jump to it.
Proceed through the HSA interview, answering "yes" to the question (if seen) "did you make any contributions to your HSA?". You will come to a screen headed by "Let's enter [name]'s HSA contributions".
On the second line where it says "any contributions you personally made", add the 2019 contribution for 2018 here.
