There are instructions for this in the facebox.js file's comments:
* The facebox function can also display an ajax page or image:
*
* jQuery.facebox({ ajax: 'remote.html' })
* jQuery.facebox({ image: 'dude.jpg' })
So, to open content.html on page load you would use the following in
your head (with the correct path to content.html):
<script type="text/javascript">
jQuery(document).ready(function($) {
jQuery.facebox({ ajax: 'content.html' })
})
</script>
Or, if you also require Facebox to be available for triggering by
anchors in the page using rel=:facebox":
<script type="text/javascript">
jQuery(document).ready(function($) {
$('a[rel*=facebox]').facebox()
jQuery.facebox({ ajax: 'content.html' })
})
</script>