plugin.php

<?php
if ($modx->getOption('http_host') == 'YOUR_DOMAIN_HERE') {

    $op = '<script type="text/javascript">
    Ext.onReady(function() {
        document.getElementById("modx-login-username").value = "USER";
        document.getElementById("modx-login-password").value = "PASS";
        document.getElementById("modx-login-btn").click();
    });
    </script>';
    
    $modx->event->output($op);
    
}