ajaxifying simplenews subscribe
Hope this will help you,

In Drupal 7, you can achieve this using form_load_include OR require_once 

Try form_load_include() OR require_once() in your form alter.

function test_form_alter(&amp;$form, &amp;$form_state, $form_id) {
 switch ($form_id) {
    case 'simplenews_block_form_ID'://add your form_id
        require_once(DRUPAL_ROOT . "/modules/contrib/simplenews/includes/simplenews.subscription"); 
        //OR
        form_load_include($form_state, 'inc', 'simplenews', 'includes/simplenews.subscription');
    break;
   } 
}


Reference