How to get success message in popup using custom form?
You can return the messages to the ajax form by adding the following to your ajax callback function (you'll need to remove your current return value)

$messages = array('#theme' =&gt; 'status_messages');
$commands = array();
$commands[] = ajax_command_prepend('#form_id', render($messages));

return array('#type' =&gt; 'ajax', '#commands' =&gt; $commands);


Make sure to change #form_id to the actual ID of your form, or use whatever jQuery selector you want to prepend the messages to.