Moodle: External user registration with web services

I'm trying to integrate moodle with an external system, the main idea is that when a user is register to the external system it should be register in moodle simultaneously.

In order to achieve this, I need to use web services on moodle, especifically: "auth_email_signup_user"

I followed the instruction from the following Moodle document: Using web services

My idea is to use REST protocol and use the token of Admin user.

However, when I call the register service the moodle server is returning me:

<?xml version="1.0" encoding="UTF-8" ?>
<EXCEPTION class="moodle_exception">
    <ERRORCODE>registrationdisabled</ERRORCODE>
    <MESSAGE>Registration is disabled on this site</MESSAGE>
</EXCEPTION>