The W-9 tax form is used for entities or individuals that are resident in the US for tax purposes. Have I Been Pwned is operated by Superlative Enterprises, an Australian proprietary limited company; therefore, the correct US tax form is a W-8BEN-E which is intended for entities that are not resident within the US for tax purposes.
Download a completed version of the W-8BEN-E form for Superlative Enterprises Pty Ltd below: