When you set your event page to "Online event", the Watch Stream page to share the online viewing URL with the attendees will automatically be enabled. From the Watch Stream page attendees will be able to access the Streaming URL as well as read the notes for the stream.

Users who ordered tickets for your event will gain access to the Watch Stream page for the event through ticket dashboard in their account, the order confirmation email as well as the event reminder email (sent automatically 24 hours before the start of the event). Users will need to login to the account they used to order their tickets to access the Watch Stream page, and users who didn't purchase tickets cannot access the Watch Stream page for your event.  

Please see Creating an event page for an online event for more information on how to register the Streaming URL and notes on the Watch Stream page of your event.