I'm curious how I would extend iron-ajax in Polymer - what element should I put in the template to show how I'm overriding it?

bankdata-ajax.html

<link rel="import" href="../bower_components/polymer/polymer.html">
<link rel="import" href="../bower_components/iron-ajax/iron-ajax.html">
<dom-module id="bankdata-ajax">
  <template>
    <iron-ajax
      id="request_courier"
      url="[[url]]"
      handle-as="json"
      last-response="{{resultingData}}">
    </iron-ajax>
  </template>
  <script>
    Polymer({
      is: 'bankdata-ajax',
      properties: {
        endpointLocation: {
          type: String,
          notify: true
        },
        url: String,
        bankId: {
          type: String,
          notify: true
        },
        resultingData: {
          type: Object,
          notify: true
        }
      },
      listeners: {
        'endpoint-location-changed': '_setUrl',
        'bank-id-changed': '_setUrl'
        'resulting-data-changed': '_deliverData'
      },
      _setUrl: function() {
        base = "http://localhost:8000/banks/";
        
        this.url = base + this.bank_id + this.endpointLocation;
        this.$.request_courier.generateRequest();
      },
      _deliverData: function() {
        // send resultingData to the template that has the bankdata-ajax
      }
    })
  </script>
</dom-module>