When creating a form, you have the option to redirect the user to a different page after they have submitted the form:
In addition to redirecting, you can also have some data about the new subscriber sent along in a query string. If you check off the “Send subscriber data to post-submission page” box, you will get something like this URL:
You can then use your new subscriber's email to customize your page with a response. In addition, you can use the subscriber_id to make API calls and get even more data about your new subscriber.
This option is also available on the post-confirmation redirect page. It will yield the same results in the query string.