Medicare has zero to do with your credit report. It’s an age or disability and Income thing. The IRS knows your income and how much you paid into Medicare over the years. The IRS is the source CMS uses for income data.
If you want to create an online account, Experian has to be thawed or unfrozen for identity verification. A Freeper posted a direct link up thread at the SS site stating so. One would think this medicare/SS stuff would be on autopilot. I've lived at the same address for 35 years. Closing in on Medicare eligibility, I would think they could send me some snail mail what I have to do if anything at all.