How can I now claim the benefits if I forgot to submit a W8?

How do I claim treaty benefits?

You must file a U.S. tax return and Form 8833 if you claim the following treaty benefits:

1. A reduction or modification in the taxation of gain or loss from the disposition of a U.S. real property interest based on a treaty.
2. A change to the source of an item of income or a deduction based on a treaty.