Custom Checkout Paramater

The following options are available to you. As a more advanced option, you can, for example, apply the following filter to change the post_id parameter and the name of the field that the JavaScript looks to fill out for you

add_filter('rup_sell_ind_posts_param_key', function($key) {    return 'custom_post_id'; // Replace 'custom_post_id' with your desired key});