Add a field into a shortcode of an extension

to customise the complete result of the shortcode, you can try this code which redefine the shortcode :

add_action("plugins_loaded", function () {

    if (!isset($GLOBALS["shortcode_tags"]["wpneo_crowdfunding_form"])) {
        return;
    }

    $original_callback = $GLOBALS["shortcode_tags"]["wpneo_crowdfunding_form"];


    add_shortcode(
          "wpneo_crowdfunding_form"
        , function ($attr, $content, $tag) use ($original_callback) {

            $original_result = $original_callback($attr, $content, $tag);

            // customise the HTML result of the form
            $original_result = str_replace(
                  "Title"
                , "A <sup>little</sup> <sub>fantasy</sub>"
                , $original_result
            );


            return $original_result;

        }
    );


});