By setting your event type as "Online Event" when creating an event page, the Online page will be enabled.
You can embed a Vimeo or YouTube video on the Watch Stream page of your online event by selecting "Embed Vimeo" or "Embed YouTube" as your streaming method on the Online page.
For detailed instructions on how to embed a video, refer to this help page: How to select/set an external streaming platform (e.g., Vimeo, YouTube) for an online event.