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.
Because of security concerns with the way widgets are built from PHP and Javascript code, creating them is restricted to administrators. While it might be possible to build a more general purpose solution that properly sanitized the various form fields (and form types, available options, etc.) I don't know of anyone that has implemented that.
