Each city, town, or agency of the state participating under the federal state agreement is authorized to impose upon each of its employees, as to services which are covered by the federal state agreement, or by any supplement thereto or modification thereof, a contribution with respect to his or her salary or wages as herein defined, equal to the amount of tax which would be imposed by § 3101 of the federal Insurance Contributions Act, 26 U.S.C. § 3101, if their services constituted employment within the meaning of that act. The obligation to make those contributions shall constitute a condition of employment by the city, town, or agency of the state with respect to the employee.