If you are in a country with a tax treaty rate for US RWHT, then you need to add a tax identification number to claim a treaty benefit. If you have a tax number from your own country (i.e. a “foreign tax identification number”), then add that into the W-8 form where specified. This is usually the number that you quote when you lodge your tax return or pay tax in your own country.
If you’re not sure what is the right number in your country, talk to a tax advisor or accountant. You may need to apply to your country’s government or tax authority. Alternately, you can apply and get a US number called an ITIN (Individual Taxpayer Identification Number).
If your country has a tax treaty with the US, there is a simple application process with the IRS that involves a form called the W-7. Assuming you’re not a US person, then you will most likely be completing the form as a “Nonresident alien required to get ITIN to claim tax treaty benefit”. However, if your country does not have a treaty with the US, then the tax identification number is not necessary as you are unable to claim a treaty benefit.
If you’re a non-US corporation and don’t have a foreign country ID number, then you have to apply for a US Employer Identification Number (EIN). As always, consult a tax advisor or accountant for assistance, or check the IRS website.