Medicare disqualifies you from making contributions to an HSA, even if you still have HDHP coverage. You should select "self only" (if you had a self-only HDHP) for January-May, and select "Medicare" for the rest of the year.
Your HSA limit for 2023 will be $2020 ($4850÷12 months x 5 months). If you contributed more than $2020, you will have to remove that excess contribution before April 15, or pay a 6% penalty.
