Hi Kyle,
if you are using the embed script method on the landing page which is to be iframed. You need to run the code after the form renders.
This can be accomplished with a callback method.
Insert this code before the marketo form embed code
<script>
function getReferrer(){
var url = (window.location != window.parent.location) ? document.referrer: document.location;
$("#LeadSource").val(url);
}
</script>
Then on the Marketo forms 2.0 embed code insert the function name after the form id value.
ie
<script src="//app-sjo.marketo.com/js/forms2/js/forms2.js"></script>
<form id="mktoForm_XXXX"></form>
<script>MktoForms2.loadForm("//app-sjo.marketo.com", "XXX-XXX-XXX", XXXX, getReferrer);</script>
I would also use a different version of jquery too
<script type='text/javascript' src='//ajax.googleapis.com/ajax/libs/jquery/1.8.2/jquery.min.js'></script>
I am unsure if Marketo supports version 1.9