JQuery :: Set Focus On The Username Field In The Form In The Fancybox Window When The Page Loads

I have a form which open in a jQuery Fancybox Window. The fancybox window opens automatically when the page loads. When it loads, I would like it to set focus on the username field in the form in the Fancybox window. I have found that this must be done when fancybox is envoked, thus I have tried the below code which does not work :

<script type="text/javascript">
jQuery(document).ready(function() {
$("#hidden_link").trigger('click');
document.getElementById('login_form.username').focus();
});
</script>

The form name is "login_form" and the field name is "username".