@kimberly3ct
In this case the Medicare web site is incorrect, or perhaps overly cautious. The IRS doesn't care when in the year the contributions are made, including retroactive contributions made before April 15 of the following year, as long as the overall eligible total is not exceeded. That's how form 8889 and instructions calculate it, you can also review publication 969.
https://www.irs.gov/forms-pubs/about-publication-969
https://www.irs.gov/forms-pubs/about-form-8889
In turbotax you enter the amount contributed (workplace contributions are captured from the W-2, not entered separately, but you would separately enter any separate out of pocket contributions). Turbotax will tell you that you have excess contributions and ask if you will remove them before the April 15 filing deadline. When you say yes, your form 8889 is adjusted. Form 8889 should report the net contributions, it does not actually report the excess and the removal, just the net. The ineligible amount is also added to your taxable wages for the year.
The HSA bank must also return any Interest or investment gains attributable to the excess contributions. for example, if you ask for a return of $1000 excess contribution, they might send back $1100. That extra income is reported on your 2025 tax return even though it was paid in 2026. Report it as bank account interest and check the box for "I did not get a 1099-INT for this interest.)
In January 2027, you will get a 1099-SA for 2026 with the distribution reported with code 2 for return of excess. This will not be taxable on your 2026 return.
