There's no preview option available on the Customize Confirmation Message step of a Calendar Event. How do I know what the page will look like?
Because there is no Preview feature or Test Drive mode for Calendar Events, the best way to know exactly how the confirmation page will display to a user is to register for the event on the front end website. If the event type is a ticketed one, then that would involve making a live transaction on a real credit card, as the test credit card number would not work.
If you would like to submit a product enhancement request for the Preview feature and/or Test Drive mode for Calendar Events please comment within the Idea Portal.