Hello @Szczepankowski, Jakub ,
Thank you for contacting Microsoft Support!
According to the documentation, it's not possible to filter the webinar object.
You can submit this feature request using this support link, which will be monitored by Microsoft team and make the enhancements to Graph API.
Hope this helps.
If the answer is helpful, please click Accept Answer and kindly upvote it. If you have any further questions about this answer, please click Comment.