Processing a subscription form with POST method?

This solution is based on a couple of WPMU Dev plugins: PopUp Pro and Mailchimp Integration. If you are WPMU Dev member then download and install them either from their Project pages or via the WPMU Dev dashboard in your WordPress admin interface.
Most of the effort is in creating a custom style for the popup. The Mailchimp Integration plugin simply provides a shortcode that can be added to the body of the popup via a TinyMCE button but we’ll cover all this later.
The first step, after installing the plugins is to configure the Mailchimp Integrationplugin.