Forgive me if I don’t fully understand what you’re trying to do, but I think one way you could do this is to use the “append form data from URL” checkbox in the right-side column. Then, on your confirmation page, as long as you have a form with an email field, it should automatically populate the email field with the visitor’s email address from the previous page so they don’t have to type it in again.
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You could add a call-to-action like “Subscribe to our blog” or something like that.
Once they hit submit, this would bascially become a separate form submission from your confirmation page, so you’d have to configure where the lead should go of course.
There’s probably an easier way to accomplish this. Just throwing out one suggestion.