Once you integrate your Zoom or GoToWebinar webinar with your account, you will be able to create workflows related to the webinar.
Use the Custom Event trigger and choose one of these options:
Webinar Attended: Once a contact attends a webinar of your choice, they will enter the workflow.
Time Spent (Minutes): If a contact spends a specific time, more or less, it all depends on your choice, they will enter the workflow.