California Code, Unemployment Insurance Code - UIC § 13029

(a) From remuneration for services performed by an employee for his or her employer which (without regard to this section) does not constitute wages, and

(b) From any other type of payment with respect to which the department finds that withholding would be appropriate under the provisions of this division, if the employer and the employee, or in the case of any other type of payment the person making and the person receiving the payment, agree to the withholding.The agreement shall be made in the form and manner as the department may by authorized regulations provide. For purposes of this division (and so much of Part 10 (commencing with Section 17001) and Part 10.2 (commencing with Section 18401) of Division 2 of the Revenue and Taxation Code as relates to this division) remuneration or other payments with respect to which the agreement is made shall be treated as if they were wages paid by an employer to an employee to the extent that the remuneration is paid or other payments are made during the period for which the agreement is in effect.