Currently you cannot create a custom URL for an event registration form. Custom forms are able to be embedded on external web pages. Currently, event registration forms cannot be embedded on external web pages on our site. This means that you cannot even create a custom URL for an external page and then embed the event form there. This is a problem for marketing and communications.
The event registration form really needs to be embedded on an external webpage. Currently, people are to click through multiple pages to just register...talk about a barrier to entry.
you can use onMessage > Settings > Website Redirects to create the custom URL to redirect to the event registration page.