Click the following link which will take you to a "raw" html page that is a starting point for your own Online Registration shell.
Once you are on that page, just view the source and copy all the html and paste that into the HTML Source view of the editor.
Note that you can have all your registrations use the DefaultShell or each Organization can specify it's own shell.
You find the DefaultShell in the your system by going to Admin > Special Content.