I think that you will have to set up a different program/event for each webinar, even if you do the form only once to get the integration to work. But each program would then be synced to 1 particular webinar within GoTo. They would each have the trigger campaign within Marketo that shows fills out form and includes the filter of webinar registered is such & such. Then the flow would be change status in progression to show as registered for that program. This would push the registrant to GoTo for just that webinar, I believe.