This GET endpoint is used to lookup whether a user with the provided email and first name exists in the MassPay system.
To use this endpoint, you need to provide the email, first_name, and internal_user_id as Query parameters in the URL.
The endpoint will then search for the user based on the provided information and return a JSON response indicating whether the user exists or not. If the user exists, the response will also contain the user's details, user_token, first_name, last_name and internal_user_id.