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If you are not a US person you complete form W8 BEN
Give Form W-8 BEN to the withholding agent or payer if you are a foreign person and you are the beneficial owner of an amount subject to withholding.
Submit Form W-8 BEN when requested by the withholding agent or payer whether or not you are claiming a reduced rate of, or exemption from, withholding.
The US company needs that so they can deduct the payment to you from their income