Organising an event? True, it’s as likely to be online these days as it is to be a physical one, but if the registration system is online, the rules for an effective promotional web page are the same. Like any landing page, it should be focused on a single call to action, in this case registration.
Obviously the primary way to do this is to make the event sound as interesting and unmissable as you can. A couple of boring bullet points isn’t enough. If you’re struggling, think about the disadvantage people might give themselves by not attending, then turn around that idea into something positive. Ensure that the essential details (date, time, venue) are apparent, and that it’s clear that it’s going to be easy to register.
If you can, think about different landing pages for different groups of potential attendees. Once you’ve designed a page, creating variations should be easy.