Solution - 1

There are few changes you will have to make to be able to display WP Email Capture form in WP Popup scheduler. First add the following in wp-email-capture/wp-email-capture.php on line 27:

function wp_email_capture_in_popup($atts) {
    $form = preg_replace("/[\n\r]/", "", wp_email_capture_form_page());
    $form = addslashes($form);
    return $form;
}

add_shortcode('wp_email_capture_popup', 'wp_email_capture_in_popup');

$popup_content = do_shortcode($popup_content);

[wp_email_capture_popup]