title: add apply coupon link to woocommerce customer payment page
body: <p>I have developed a system to create orders programmatically in woocommerce.  However when a customer clicks on the link to go to their payment page it does not show the 'Have a Coupon' input box.  I haven't adjusted any of the templates.</p>
  
  <p>Any ideas on how I can get the coupon input box to appear on the customer payment page? </p>
  
  <p>Location I am talking about is:</p>
  
  <pre><code>&lt;mywebsite&gt;/checkout/order-pay/&lt;order_id&gt;/?pay_for_order=true&amp;key=&lt;order_key&gt;
  </code></pre>
  
  <p>Thank you for any help with this, I am struggling with it</p>