This page contains forms that you can customize according to which kind of information you want your users to provide. There are existing forms here as well as a "Create new form" button.
The "Actions" button far to the right of the screen allows you to
- preview of the selected form
- modify the form
- delete the form
...
Code strings look like this:
<input name="group" type="hidden" value="1">
<input name="list" type="hidden" value="2">
In the example above, the user will be subscribed to Group 1 in List 2.
To retrieve Group and List codes, go to Settings > Developer's corner > Codes Table.