Is it possible for 3rd Party Script to Create Users
And also it's very simple to create users in D8 by program:

$newuser = \Drupal\user\Entity\User::create([
          'name' =&gt; $name,
          'pass' =&gt; $pass,
          'mail' =&gt; $mail, 
          'status' =&gt; 1,  // or 0 if inactive
          'init' =&gt; $initmail,
          'created' =&gt; time(),
          'login' =&gt; 0,
          'access' =&gt; 0,
        ]);
      $newuser-&gt;addRole("myrole");
      $rc = $newuser-&gt;save();
      if ($rc == SAVED_NEW) {
    ...whatever...
    }


For bootstrapping external scripts, you can look i.e. here: Bootstrap from external script