MailChimp Single Opt-In Widget
The embedded forms provided by MailChimp all enforce double opt-in. This means that once a subscriber submits their email address they are taken to a page off the site to indicate their willingness to add their name to the mailing list. MailChimp allows single opt-in by letting site visitors submit their email addresses through their API wrapper. This MailChimp Single Opt-In Widget Plugin installs a widget that uses AJAX to submit an address through the MailChimp API wrapper. Thus site owners can easily let site visitors subscribe to their mailing list without having to go through the double opt-in process.
Simply install the plugin at your site, then go to Settings->Mailchimp Single Opt-In where you can enter your MailChimp API key and List ID along with any text for the field label, placeholder, subscribe button and thank you message. Then go to Appearance->Widgets and drag and drop the MailChimp Single Opt-In Widget to the sidebar where you want it to appear. You can also insert it into a page or post using this short code:
[display-mc-form]
Or you can add it to your PHP code using:
<?php echo do_shortcode('[display-mc-form]'); ?>
The widget and the short code forms are slightly different so that both will work properly
if they appear on the same page.