If you need to get the webinar registration link, this will need to be obtained through the onlineMeetings endpoint, and it should be noted that this version is a beta version, see the link for more information.
https://learn.microsoft.com/en-us/graph/api/meetingregistration-get?view=graph-rest-beta&tabs=http
If you want to get a webinar registration record that represents an attendee, you can view the link to get a record of the registration for all attendees.
https://learn.microsoft.com/en-us/graph/api/resources/virtualeventregistration?view=graph-rest-1.0
Hope this helps.
If the reply is helpful, please click Accept Answer and kindly upvote it. If you have additional questions about this answer, please click Comment.