The problem is that most people don't understand their Medicare effective date. I just helped someone a couple days ago who went on Medicare in May due to disability, but the effective date was 2/1/21, so they became ineligible for an HSA earlier than they expected and had an excess contribution.
If you understand the Medicare rules are are confident that your effective date will be 7/1/22, then you can contribute 6 months worth (6/12ths) of the annual maximum (depending on if you have a single or family HDHP).
Also be aware that in many cases, your spouse may be able to contribute to their own HSA if they are covered by your policy, even if they are not the named insured, and even after you are ineligible. HSAs can be opened at many banks and brokers, they don't have to be sponsored by an employer.
