Being enrolled in part A makes you ineligible to contribute to an HSA. Assuming your enrollment is backdated to August 1, 2024 (based on your story) then for 2024, your maximum eligibility to contribute to an HSA if you have single coverage is:
$4150 + $1000 catch-up ÷ 12 months x 7 months = $3004.
If you have a family HDHP, your maximum contribution is $5425 (calculated the same way).
You will not be eligible to contribute in 2025 or the future.
If you are married and your spouse is covered by your HDHP and not covered by Medicare yet, they are likely eligible to contribute to an HSA in their own name, they can set one up at many private banks or brokers.
You must remove any excess contributions from your HSA by April 15, 2025. This is a special procedure, not a regular withdrawal, so contact the bank first. The bank must also return any earnings that are attributable to the excess contributions (they know how to do this) and those earnings are taxable "miscellaneous other income" on your 2024 return, even if the earnings are not returned to you until 2025.
