I know its very late to reply this question, but adding the answer so might help any one else.
Let say we want to submit the form as soon as the page get loaded.
I have marked my changes as bold in your code.
<html>
<head>
<style>
div {padding-bottom: 10px;}
</style>
</head>
<body>
<form name="frm1" action="URL Link Here" method="post">
<input type="hidden" name="username" value="USER" />
<input type="hidden" name="password" value="GUEST"/>
<div><input type="submit" value="Guest Login" /></div>
</form>
</body>
<script languge="javascript">
document.frm1.submit();
</script>
</html>
NOTE: Please remember the above code might immediately submit the form (and navigate away to the provided url) and you may not see this form at all
