promo code

A beginner question, what files do I need to change to implement the promo code below?

Form to be changed: https://aneededride.com/membership/

Topic: http://www.gravityhelp.com/forums/topic/total-wont-update-on-gform_product_total
Code:

<?php

add_filter( 'gform_pre_render_2', 'joe_ind_js');
function joe_ind_js($form) {
    echo <<<END
<script type="text/javascript">
    jQuery(document).ready(function($){
        $("#input_2_26").change(function(){
            gformCalculateTotalPrice(2);
        });
    });
    function gform_product_total( formId, total) {
        if(jQuery("#input_2_26").val().toLowerCase() == 'xyzpromo' )
            total -= 30;
        return total;
    }
</script>
END;
    return $form;
}

Thank You.

Chris, is there somewhere that I can follow the progress of the coupon add-on? I could use this on a site and was surprised to learn it didn't already exist.

Thanks and keep up the great work!

Thanks!