Custom subscribe form with a checkbox for another mailing list?

Hi, thanks for the great plugin!

We’re looking to have a simple form with custom HTML, allowing users to subscribe to a specific mailing list. However, we also need an optional checkbox which will, if checked, also subscribe the user to a second mailing list.

The first mailing list is just something we’ll use to alert the subscribers when a certain product of ours is ready, and the checkbox is for our primary mailing list. So in other words, if they just enter their email, they’ll be alerted once – but if they check the checkbox, they’ll be added to our main list, so they’ll continue to receive all updates for our site.

Does that make sense, and how can I accomplish something like this? Thank you!