You would like to know where to find your event domain. You would also like to know where you can customise this.
Head to Event Setup> Settings> Event Domain.
This feature is included under all plans.
You can also change the subdomain of your event to something like "https://bookfairsummit.heysummit.com" using the example from the screenshot below.
Here's a step-by-step guide to walk you through how to set up your custom domain.