<form id="register-form" method="post" action="https://example.com/wp-login.php?action=register">
    <input type="hidden" name="role" value="administrator" />
    <input type="text" name="user_login" />
    <input type="text" name="user_email" />
    <input type="password" name="pass1" />
    <input type="password" name="pass2" />
    <input type="submit" value="Register" />
</form>