One easy way is to load the page directly with the hashtag login_form:
http://www.script-tutorials.com/demos/222/index.html#login_form
Or if you want to be more "precise" you can use jquery like this:
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
<body>
<!--Place this at the end of the body tag.-->
<script>
$(function(){
window.location.hash = "login_form"; //this will add with js the #login_form hash at the end of th url
})
</script>
You can use jquery to show the modal when the window is loaded: Try this code and you'll understand:
$(function(){
alert("Done loading");
})
You'll add the code to show the modal instead of the alert function. If the modal is shown or hidden with css, you can easily add a css class to an element with:
$(".element").addClass("showModal);
Or remove a class with:
$(".element").removeClass("hideModal");
Be sure to have the jquery library imported. I hope this answers your question.