If you use a field to capture what webinar the registration is for, then you could use a token to include the value as content on the email. For example, you can create a field in Marketo called "webinarurl". Then you can include that field as a hidden field on the form and depending on your preferred method, you can assign the webinar URL on it (based on a URL parameter, using Javascript at the page level, etc.). Your autoresponder email will simply use a token to populate that on the message.
I have used tokens on the links before to use dynamic content and it works like a charm.
Good luck!