PayPal button - Add custom property to be sent back from IPN listener

I need to add a custom key to my PayPal button that gets returned with a value from the IPN Listener. My current PayPal code is below. Can I add a custom key/property to the src url or the paypal.Buttons script?

<!DOCTYPE html>
<html>
  <head>
    <base target="_blank" />

     <script src="https://www.paypal.com/sdk/js?client-id=MyID_here&currency=USD" 
       data-sdk-integration-source="button-factory"></script>
     <script>
      paypal.Buttons({
        style: {
            shape: 'rect',
            color: 'gold',
            layout: 'vertical',
            label: 'paypal',

        },
        createOrder: function(data, actions) {
            return actions.order.create({
                purchase_units: [{
                    amount: {
                        value: '9.99'
                    }
                }]
            });
        },
        onApprove: function(data, actions) {
            return actions.order.capture().then(function(details) {
                alert('Transaction completed by ' + details.payer.name.given_name + '!');
            });
        }
    }).render('#paypal-button-container');
  </script>
</head>

<body>
  <div id="paypal-button-container"></div>

</body>
</html>

Solution

In the purchase_units object, you can supply a custom_id

It is documented here: https://developer.paypal.com/docs/api/orders/v2/#definition-purchase_unit