I assume this in on your own website, not someone else's?
It doesn't have to be a robot that emulates an admin user ...
It needs to be a PHP script that you create to INSERT or UPDATE MySQL tables.
You just need to connect to the MySQL database and INSERT into the wp_user table
and the wp_usermeta table.
You'll need to know things about the user, like username and password.
Will the user create their own password ... how will you know what to put there?
If you just make-up a random password, you'll have to somehow convey that back to the person logging-in.
There could be some real security issues here with a PHP script that adds a user.
Why would you want to have something like this? It's very dangerous.
NOTE: This is also the method to create an admin person in case you only had
one admin and the password was forgotten or lost. You can also do this manually
using your webhost's MySQL control panel (mysql editor). My opinion ... creating
a PHP script to add users is a stupid idea.