Hey Lilo – thanks for reaching out!
In order to add a new email address or phone number to an existing Stytch User object, you’ll need to pass the new email address or phone number into the respective /send endpoint for the authentication method of your choice (for example, the /magic_links/email/send endpoint).
If you specify the existing user’s Stytch user_id while calling the /send endpoint, the new email address or phone number will be added to the existing User object upon successful authentication. We require this process to guard against an account takeover vulnerability.
Please let us know if you have any follow-up questions about this, and we’ll be happy to help!