If you already have Medicare Part A, the next step is to complete the CMS-40B form from the Social Security Administration.

Specifically, the SSA requires forms like the CMS-40B and CMS-L564 if you're applying ... include medical documentation and any previous Social Security Administration (SSA) decisions about ...

If you want to enroll in Medicare Part B, you can do so when you first become eligible for Medicare via the Social Security Administration ... you can use form CMS-40B to sign up for Part B ...