Hi Lauren,
You're talking about the personalized url that would be included in your email using the {{member.webinar url}} token, correct?
In order to make this token work, it must be used in an email that is being sent through a flow action in a smart campaign that is a child asset of the program that is synced with your webinar provider. The lead must also have a status of Registered within that program, which is what tells Marketo to push their information to the webinar provider.
The best way to test this is to go through the registration process yourself, as if you were one of your attendees.
John