42 CFR §407.22

1. (a)A request for enrollment is required of an individual who meets the eligibility requirements of § 407.10 and desires SMI, if the individual—
   1. (1)Is not entitled to hospital insurance;
   2. (2)Has previously declined enrollment in SMI;
   3. (3)Has had a previous period of SMI entitlement which terminated;
   4. (4)Resides in Puerto Rico or outside the United States; or
   5. (5)Is enrolling or reenrolling during a special enrollment period under § 407.20.
2. (b)A request for enrollment under paragraph (a) of this section must:
   1. (1)Be signed by the individual or someone acting in his or her behalf; and
   2. (2)Be filed with SSA or CMS during the initial enrollment period, a general enrollment period, or a special enrollment period as provided in § 407.20.