42 CFR § 406.7 - Forms to apply for entitlement under Medicare Part A.
The following forms, available free of charge by mail from CMS or at any Social Security branch or district office, are used to apply for Medicare entitlement under the circumstances indicated:
CMS-18-F-5 - Application for Hospital Insurance Entitlement. (For use by individuals who are not eligible for retirement benefits under Title II of the Social Security Act or under the Railroad Retirement Act. This form may also be used for enrollment in the supplementary medical insurance program.)
CMS-43 - Application for Health Insurance Benefits under Medicare for Individuals with End Stage Renal Disease (ESRD). (An initial application for entitlement by individuals with ESRD).