This plugin helps you grow your MailChimp lists and write better newsletters through various methods. You can create good looking opt-in forms or integrate with any existing form on your site, like your comment, contact or checkout form.

1. Install the plugin and activate it https://wordpress.org/plugins/mailchimp-for-wp/
2. Go to Dashboard > MailChimp > Form and paste this:

   <div class="subscribe-widget">
     <h4>Newsletter!</h4>
     <p>Sign up and receive the latest Themes via email.</p>
     <input type="email" name="EMAIL" placeholder="Your email address" required />
     <input type="submit" value="Subscribe" />
   </div>