Code §1473(1)(A) provides that the term “withholdable payment” means:
(i) any payment of interest (including any original issue discount), dividends, rents, salaries, wages, premiums, annuities, compensations, remunerations, emoluments, and other fixed or determinable annual or periodical gains, profits, and income, if such payment is from sources within the United States, and
(ii) any gross proceeds from the sale or other disposition of any property of a type which can produce interest or dividends from sources within the United States.
See also Treas. Reg. §1.1473-1(a)(1).
Under both the statute and the regulations, in order for a payment to be a withholdable payment, it must be US source income (which would mean that you were performing the services from within the US)
Note line 2 here: