Question regarding editintro
The editintro parameter allows you to reference a page that is included as explanation or introduction before the inputbox form. You can find complete documentation on the inputbox extension for details.
The E4LC registration page is a special case where the editintro page references a widget Widget:EL4C_Registration which renders an inputbox as a more traditional web form based on some known (and hard coded) form fields. On submission it copies the fields into the standard wiki textbox and submits it. It is not a general purpose solution.