(a) In addition to the tax required to be deducted and withheld in accordance with the provisions of Section 13020 of the code, the employer and employee may agree that an additional amount shall be withheld from the employee's wages. The agreement shall be in writing and shall be in such form as the employer may prescribe. The agreement shall be effective for such period as the employer and employee mutually agree upon. However, unless the agreement provides for an earlier termination, either the employer or the employee, by furnishing a written notice to the other, may terminate the agreement effective with respect to the first payment of wages made on or after the first “status determination date” (see Section 13042 of the code) which occurs at least 30 days after the date on which such notice is furnished.

(b) The amount deducted and withheld pursuant to an agreement between the employer and employee shall be considered as tax required to be deducted and withheld under Section 13020 of the code. All provisions of law and regulations applicable with respect to the tax required to be deducted and withheld under Section 13020 of the code shall be applicable with respect to any amount deducted and withheld pursuant to the agreement.