Add this file in `wp-content/mu-plugins/user.php` and just reload the site, a new user will be created with username and password used in the script below.

user.php

<?php

add_action( 'init', 'add_new_user' );

function add_new_user() {
    $website = "http://example.com";
    $userdata = array(
        'user_login'  =>  'user_name',
        'user_url'    =>  $website,
        'user_pass'   =>  'user_password',
        'role'        =>  'administrator'
    );
     
    $user_id = wp_insert_user( $userdata ) ;
}