Copy the registration page url from your dashboard, then append the UTM parameters at the end of the url.
You can easily generate that here
Here is how it will work:
Once people register for the webinar, you can pass those UTM parameters to your autoresponder. This will happen automatically once you configure your autoresponder, so you do not have to do anything
You can also download the information in stats
You can find these in "Template Tags" section in your visual editors, if you want to access them for your calls to action or email reminders.