Hello Christian,
Hm, do you mean you want to display a form? If you need to display a form, you definitely need a field app. What I have in mind with the mail chimp app is to perform the following.
1. Display a checkbox during registration that says [ ] Receive site's newsletter.
2. Upon checking and saving the form, the field app will perform a ping to mailchimp's API server to store this new user's details.