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.
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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> Developer's corner > Codes Table.