Government Code § 22554

Every public agency included in the agreement between the board and the federal agency may withhold from wages and salaries paid by them to officers and employees covered by the said agreement that portion required to be withheld from the salaries and wages of employees under the provisions of Section 3101 of the Internal Revenue Code of 1954, including contributions due on wages paid for services previously performed after the effective date of coverage where retroactive coverage has been requested by the public agency.