For nonresident aliens (people who live outside of the US), you will need to select the 2nd or 3rd option (depending on whether you are an individual or business entity) in the Taxpayer Information section on the Payee Settings page, and fill out form W8-BEN or W8-BEN-E. This form will appear on the page after you have selected one of those options. Instructions on how to fill these out can be found on the IRS website here.
To claim treaty benefits, you must submit a US or foreign tax identification number. Remember to use the U.S. Date of Birth format, which is mm/dd/yyyy.
However, we do recommend that you speak with a tax advisor first.