You must request a waiver of the five-year requirement from OPM. The steps you must take are given in the FEHB Handbook at
Waiver of 5-Year Enrollment Requirement - Waiver of 5-Year Enrollment Requirement.
If your agency has buyout authority, you may not need to write to the OPM. If you think you might qualify for a waiver of the 5-year coverage requirement, contact your Human Resources Office for information. If you meet the requirements, your agency will attach a memorandum to your retirement application stating that you meet the requirements for waiver by the OPM.