Who is require to withhold? • Withholding Agent.The first step is to have the contractor complete Form W9, Request for Taxpayer Identification Number and Certification. Learn more about taxes and payment methods you need to be aware of to make the most out of working with contractors abroad. Information for committees on prohibited sources of contributions from foreign nationals. Are there consequences for the contractor (me) if I never filled out a W-8 for my US client? The amount you can exclude is calculated based on a formula tied to the current year's foreign earned income exclusion amount.