I took a look at your page and, correct me if I'm wrong, but you have the same presentation scheduled at four different times.
I suggest instead of four pages, you condense this into one. You can do this by adding a drop down field to the form that allows the user to pick which sessions they would like to attend. This way you are still able to have hidden source field on the form.
Are you using a Marketo webinar connector? If so, you will still be able to separate the leads by campaign based on their form submission.
Please let me know if this all makes sense or if you have any questions.