Every employee of the state whose services are covered by an agreement entered into under sections 68-603 and 68-604 shall be required to pay for the period of such coverage, contributions, with respect to wages, as defined in section 68-602, equal to the amount of tax which would be imposed by the Federal Insurance Contributions Act if such services constituted employment within the meaning of that act. Such liability shall arise in consideration of the employee’s retention in the service of the state, or his or her entry upon such service, after the enactment of sections 68-601 to 68-631.