To my knowledge currently this is not possible.
There is an active Teams uservoice thread about this feature: Create a template for the Teams Webinar registration form
Please consider upvoting or leaving a comment to submit your feedback to Microsoft.
Thanks for your understanding.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.