Submit W-8ECI tax form

This POST endpoint is used to submit a W-8ECI tax form for a specific user identified by their user_token. The W-8ECI form is used by foreign persons to certify that income received is effectively connected with the conduct of a trade or business within the United States. 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-8ECI form data in the request body.