Tax Law, § 671(a)(2)
An employee and his employer may enter into an agreement, under section 671(a)(2) of the Tax Law, to provide for withholding of New York State personal income tax upon payments made as remuneration for services performed by an employee for his employer which do not constitute wages pursuant to section 171.3 of this Part, but which are includible in the New York adjusted gross income of the employee. Such an agreement must be applicable to all such amounts paid by the employer to the employee. The amount deducted and withheld pursuant to such an agreement between the employer and employee is to be determined under the rules contained in section 671(a)(1) of the Tax Law and the regulations set forth in this Part.
If accepted by the employer as provided in paragraph (3) of this subdivision, the request must be attached to, and will constitute part of, the employee's Employee's Withholding Allowance Certificate (form IT-2104). An employee who furnishes such employee's employer a request for withholding of New York State personal income tax, as set forth in this section, must also furnish such employer with a completed form IT-2104 if such employee does not already have a form IT-2104 in effect with such employer.
N.Y. Comp. Codes R. & Regs. Tit. 20 § 171.7