The link above points to a TurboTax help screen, but it seems that the text at that link is somewhat outdated. In particular, the following no longer seems appropriate:
"
But what if I made an HSA contribution that wasn't reported on my W-2?
We'll ask about this uncommon situation when you step through the Medical section. Look for the questions Were HSA contributions made in 2018? or Do you need to make any adjustments to your HSA contribution amount? When you answer Yes, we'll open a screen where you can enter your contribution."
The problem is that these questions don't exist any longer (if they ever did).
So, navigate to Deductions & Credits->Medical->HSA, MSA Contributions and click on Start or Revisit. If the next screen shows you the HSA Summary, then click on Edit.
Indicate that you have an HSA, then continue through the dialogue asking for your 1099-SA (if you received one). Then TurboTax says "Let's work on [name]'s HSA". Go through this dialogue until you see the screen with the heading of "Let's enter [name]'s HSA contributions". On the second line of this screen ("Any contributions you personally made (not through your employer)"), enter the $500 ONLY IF you made the contribution directly to the HSA and not through your employer. This is what the link above is referring to.
If, however, you made through $500 contribution through your employer (i.e., the employer agreed to withhold an extra $500 in 2019 for 2018), then you have to enter the $500 on a different screen. Leave the entry on this screen blank and continue.
The next screen will probably ask "Did your employer tell you about any other contributions?" Answer "yes". Clicking on Yes will cause three more lines to appear. You want to enter the $500 on the line that states "Employer and payroll contributions made in 2019 for tax year 2018".
You likely want to use the first option ("Any contributions you personally made (not through your employer)"), but we don't know how you made the contribution, hence the excessive description ;-).
P.S. It is critical that you told the HSA administrator that the $500 contribution was for 2018; otherwise, they would assume that it is for 2019 (the current year), and their paperwork which is reported to the IRS would no longer match what your return indicates.
