title: add hidden input field to woocommerce checkout to pass data to payment gateway
body: <p>I have a store running WooCommerce and the client needs us to pass a variable to the payment gateway (WorldPay) that is then included in an payment confirmation email sent form the gateway to the client.</p>
  
  <p>WorldPay say i just need to add this to the form:</p>
  
  <pre><code>&lt;input type="hidden" value="myValue" name="M_myName" /&gt;
  </code></pre>
  
  <p>After a lot of research I figured out how to create an array of the SKUs for each product in the cart. I have created a function that will grab each product SKU and add it to an array. Then outputs this array to replace "myValue" in the above.</p>
  
  <p>My question is where do I need to put the &lt; input >? I have added it to woocommerce > checkout > review-order.php, but this doesn't seem to work. It shows in the front-end but doesn't get passed the WorldPay.</p>
  
  <p>Or would I need to add it to an existing action? I found <a href="https://stackoverflow.com/questions/11938974/woocommerce-form-field-and-hidden-fields">this</a> which seems to be talking about the similar issue – but then, where would I need to inject the output of that action, in which template?</p>
  
  <p>Thanks for any advice.</p>