38 CFR §21.5050

1. (a)An individual, who is otherwise eligible to become a participant, must apply to the Service Department under which he or she serves upon forms prescribed by the Service Department and/or Secretary of Defense.
2. (b)No application to participate may be made before entry upon active duty.
3. (c)Each application must be submitted in time to permit the Service Department to make the required deduction from the individual's military pay for at least 1 month before the applicant's discharge or release from active duty.