PayPal: How to pass customer email when creating an order?

Using https://developer.paypal.com/docs/checkout/ I create an order:

<script>
  paypal.Buttons({
    createOrder: function(data, actions) {
      // Set up the transaction
      return actions.order.create({
        purchase_units: [{
          amount: {
            value: '0.01'
          }
        }]
      });
    }
  }).render('#paypal-button-container');
</script>

I want to be able to pass customer email such a way, so when on server side I got notified about successfull transaction completed thru IPN or webhook - being able to get this same email directly or by making additional call to PayPal API?

It can be done by not using express checkout and specifying custom fields. But how to do it via jscript API in express checkout? didn't find appropriate fields in their API docs.