Wraps Stytch's send via email API endpoint. Call this method to send an email passcode to existing users.
This method is also used when you need to add a phone number to an existing Stytch User. If there is a currently valid Stytch session, and the user inputs a phone number that does not match one on their Stytch User object, upon successful authentication the new phone number will be added to the existing user. Note, this does expose a potential account enumeration vector, see our article on preventing account enumeration for more details.