In the members grid inside the event, you can add a column "Webinar URL" to see the webinar url that was created when the lead registered. In summary, there are a few things you can do:
1. Use the new calendar file feature that will resolve the member.webinar url token. This way it will be in the calendar invite!
2. Resend the confirmation email to the lead and the member.webinar url link will populate - do this as a single flow action or as reminder before the webinar