<?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);
}