Have you tried sending the email to you directly through a hard send? I have heard that the token will not populate if you do not have right status. I would try adding yourself to the program making sure your status is registered and sending the email through a smart campaign with "email address is "your email" and flow being "send email - your confirmation email with token"
What webinar platform are you using? GotoWebinar/Webex/On24?
Two requirements for this:
1. Like Ariel said, you have to send the email using a smart campaign to a real lead - and (if I remember correctly) the smart campaign has to be in the program. You can not test this using a sample email.
2. I'm guessing you already know this, but to be certain: the test lead you send to has to be registered for the webinar, and the program in Marketo has to be synced with the webinar provider. If it's not, Marketo won't have done the API call to the webinar provider to get the value it needs for that webinar url token.