Hi Elisabeth,
You can either enable the waitlist function by going to Event Details > General > Event Configuration > Check off waitlist, or you can customize the message that says "This event is closed. If you have any questions, please contact the planner directly." You can also add custom text to the Waitlist page if you want to include a hyperlink to an external site there.
To customize the message, go to Website & Registration > Website > Language Management > Search for "closed" > Edit the Custom Text to include a hyperlink of a webpage you'd like to direct people to.
You could also hide the Register button from the Summary page entirely, then put in text on the Summary page that includes a hyperlink to where you'd like to redirect registrants to.
Best,
Julia