Submit W-8BEN tax form

This POST endpoint is used to submit a W-8BEN tax form for a specific user identified by their user_token. The W-8BEN form is used by foreign individuals to certify their foreign status and claim tax treaty benefits (if applicable). Basic information (name, address, phone) is retrieved from the user's existing profile.
To use this endpoint, you need to provide the user_token as a parameter in the URL Path and the W-8BEN form data in the request body.