title: paypal send custom field with express checkout
body: <p>I want to send a custom field in paypal's express checkout so I can receive the same when the IPN is sent back from paypal. This is not clearly defined in the documentation and am stuck here. here is my code: </p>
  
  <pre><code> paypal.Button.render({
  
                  env: 'production', // sandbox | production
  
                  // PayPal Client IDs - replace with your own
                  // Create a PayPal app: https://developer.paypal.com/developer/applications/create
                  client: {
                      sandbox:    'my sandbox client ID here',
                      production: 'my client ID here'
                  },
  
                  // Show the buyer a 'Pay Now' button in the checkout flow
                  commit: true,
  
                  // payment() is called when the button is clicked
                  payment: function(data, actions) {
  
                      // Make a call to the REST api to create the payment
                      return actions.payment.create({
                          payment: {
                              transactions: [
                                  {
                                      amount: { total: '1', currency: 'GBP' }
                                  }
                              ]
                          }
                      });
                  },
  
                  // onAuthorize() is called when the buyer approves the payment
                  onAuthorize: function(data, actions) {
  
                      // Make a call to the REST api to execute the payment
                      return actions.payment.execute().then(function() {
                          console.log(data);
  
                          //my action here
  
                      });
                  }
  
              }, '#paypal-button-container');
  </code></pre>
  
  <p>I can only send the amount and currency but can't figure out how to send the custom field.</p>