This POST endpoint is used to submit a W-9 tax form for a specific user identified by their user_token. Basic information (name, address, phone) is retrieved from the user's existing profile.
The tax provider's W-9 API only accepts a US address: the user's stored state must be a US state and the postal code a 5 or 9 digit ZIP, otherwise the request is rejected with 400. For a U.S. person whose only address is outside the US (for example a U.S. citizen living abroad), collect the W-9 through the hosted tax interview link (GET /payout/USER_TOKEN/tax), which accepts a foreign address and uploaded W-9 PDFs.