Webinaris offers you the possibility to store tracking codes in different places in order to create custom or lookalike audiences on Facebook:
On the landing page
To capture all those who have accessed your landing page
On the thank you page
To capture all those who have signed up for your webinar
In the webinar room (as action)
To capture all those who participated in the webinar
In many cases, the URL of the page is used to select the target group. This is not possible with Webinaris, because the URL of the thank you page and the webinar room are different for each participant.
Instead you have to use Events. This means that when you add a Facebook Tracking pixel to the Thank You page, for example, you add an event that exactly says that the Thank You page has been accessed. In Facebook, you then don't select a URL to create the custom / lookalike audience, you select the event.
Please find more information here: https://developers.facebook.com/docs/facebook-pixel/api-reference