How to send custom user input field with Paypal Subscription

I am using Paypal subscription wizard to generate a javascript snippet for monthly recurring payments. https://developer.paypal.com/docs/subscriptions/

The javascript looks as following (modified senstive info):

<div id="paypal-button-container"></div>
<script src="https://www.paypal.com/sdk/js?client-id=esdasdcgmGsadsad34324hvfD7boV-2PMY34324234234234R7MjasdAUhH_qbIy4A&vault=true&intent=subscription" data-sdk-integration-source="button-factory"></script>
<script>
  paypal.Buttons({
      style: {
          shape: 'pill',
          color: 'blue',
          layout: 'vertical',
          label: 'paypal'
      },
      createSubscription: function(data, actions) {
        return actions.subscription.create({
          'plan_id': 'P-aaaaaaaaaaaaaaaaaa'
        });
      },
      onApprove: function(data, actions) {
        alert(data.subscriptionID);
      }
  }).render('#paypal-button-container');
</script>

I added an input text above paypal subscription buttons:

<input type="text" id="your_unique_username" value="Enter your user name">

All I am trying to do is pass the value the user placed in the input text object to paypal subscription and receive that value via email which i receive on every subscription or even better at paypal subscription dashboard where you see subscribed users.

Seems like a pretty straight forward feature to have, but haven't found any examples. What would be the best method to achieve that?

Solution

Send a custom_id variable, set to the value of an expression that will be evaluated when the button is clicked.

'plan_id': 'P-aaaaaaaaaaaaaaaaaa',
'custom_id': document.getElementById('your_unique_username').value