I deposited money into a private HSA account before the deadline in 2019 - to be applied for 2018 tax year. Where do I record this?

[BMcCalpin]

What is a "private HSA account"? Is it different than an HSA account organized under Section 223 of the US Tax Code?

[BMcCalpin]

If it is a normal HSA account after all, go to the HSA interview Search for "hsa" (lower case, without the double quotes) and jump to the result.

Indicate that you have an HSA and then go through the interview.

When you see the screen headed by "Let's enter [name]'s HSA contributions" (see screenshot below), then add your 2019 contribution for 2018 on the second line, the one that refers to HSA contributions that you "personally" made.