Ability to pass random custom fields to Mailchimp subscribe form. Fields are registered with filter call (example in the content here).

[appscreo]

add_filter('essb_custom_subscribe_form_fields', function() {
   $r = array(
       'fname' => array('display' => 'First Name', 'type' => 'input', 'map' => 'FNAME', 'required' => true),
       'lname' => array('display' => 'Last Name', 'type' => 'input', 'map' => 'LNAME', 'required' => true),           
       'phone' => array('display' => 'Phone', 'type' => 'input', 'map' => 'PHONE', 'required' => true),
       'address' => array('display' => 'Address', 'type' => 'input', 'map' => 'ADDRESS', 'required' => true)
   ); 

   return $r;
});