Adding a “Subscribe to Newsletter” checkbox or radio button to an existing contact form is incredibly painless using the MailChimp API. Include one library file, and you’ll have access to all functions the API has to offer. In this example, I added a Yes/No radio selection. You could easily change this to be a checkbox. When "Yes" is selected, and the form is submitted, the contact form does its thing, and then the user is added to a MailChimp List.