Alter Add to Cart form POST data provided by #action I am going to be phasing out Drupal Commerce on our website in favor of a new custom built application that is hosted at another URL. Without completely rebuilding everything I am trying to override the Add to Cart form to redirect with POST data to the new app. 

Using $form['#action'] I can redirect properly and dumping out shows something like this:

array(7) {
    ["quantity"]=&gt;
    string(1) "2"
    ["product_id"]=&gt;
    string(4) "8566"
    ["form_build_id"]=&gt;
    string(48) "form--90fc5Yp9qgp4CVFJlq03jfEZyYBkAhDX-j7E5g9oBQ"
    ["form_token"]=&gt;
    string(43) "T08Fl1dNHuI4BCZ4JuLNwxbw6gUTOfnodbc1zcQMR4E"
    ["form_id"]=&gt;
    string(35) "commerce_cart_add_to_cart_form_8566"
    ["field_are_you_a_product"]=&gt;
    array(1) {
        [0]=&gt;
        array(1) {
            ["field_are_you_a_"]=&gt;
            array(1) {
                ["und"]=&gt;
                string(3) "yes"
            }
        }
    }
    ["op"]=&gt;
        string(11) "Add to cart"


This is helpful but the product_id is useless as I need more info, like the SKU or a custom ID field I have on the product level. I can't seem to get this to provide me with the product info I need. 

I initially tried using a custom submit handler instead but that didn't work. drupal_http_request isn't right as it just sends post data, does actually redirect with post data like action does. 

So since setting the action works great, how would I alter this form to send more information during that step within hook_form_alter?
You can add other fields to your form as hiden, in the hook_form_alter for example add SKU field as hidden.

  $form['sky'] = [
    '#type'  =&gt; 'hidden',
    '#value' =&gt; YOUR_VALUE,
  ];


Hidden fields will submitted like others.