I think Lead Ads for a webinar could be the wrong choice, it’s quite limited what you can do with the leads you are generating. but a couple of things might be:
If you are using Clickfunnels (Actionetics) I would add them to a list, and let Clickfunnels Sync those leads with my webinar platform (WebinarJam or whatever you are using).
If you want to integrate them directly into your webinar platform (not recommended), you can use Zapier or similar services that allow the automation of such task.
To do that automatically I think you’ll have to use something like Zapier, there are alternatives to Zapier. But it can integrate with almost anything that has an API. so you should go with it.