Hey Lonnie,
Can you explain to me how you use the form? Are you embedding it on different websites?

In my case I have a website that functions like a business directory and I want to be able to track the business that generated the signup for the city newsletter. It is one mailing list, but I need to track the number of signups for each business in order to credit them with meeting their signup quota.

But this would also be an excellent feature for tracking the page effectiveness for any website. One could pass a custom field back and track the signup ratio of the pages on their websites to measure how well each page converts.

I can think of other uses as well... during trying to find out how to do this, I saw where many people have this need for many reasons. It would truly set your component apart from any other out there. No Joomla component has this feature. And, unfortunately, while the embedded HTML MailChimp provides does exactly this... the javascript they use has too many conflicts to be useful on a professional site.

Thanks!
Lonnie