Submit W-9 tax form

This POST endpoint is used to submit a W-9 tax form for a specific user identified by their user_token. The W-9 form is used by U.S. persons (including resident aliens) to provide their taxpayer identification number (TIN) to the requester. 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-9 form data in the request body.