Such referendum shall comply with the conditions set out in section 218(d)(3) of the Social Security Act, and: (1) It shall be by secret written ballot upon the question of whether service in positions covered by such retirement system should be excluded from or included under the state agreement; (2) an opportunity to vote in such referendum shall be given, and shall be limited, to eligible employees; (3) not less than ninety days’ notice of such referendum shall be given to all such employees; and (4) such referendum shall be conducted under the supervision of the Governor, or an agency or individual designated by him.