title: Custom event parameters not being sent
body: <p>First off: I'm very new to Google Analytics!</p>
  
  <p>I'm trying to use GA to track a custom event with some custom parameters, but I'm not doing very well. I've set up <code>gtag.js</code> according to <a href="https://developers.google.com/analytics/devguides/collection/gtagjs/" rel="nofollow noreferrer">this</a> and it is working if I only set the following parameters:</p>
  
  <pre><code>function trackOutboundLink(url) {
          gtag('event', 'click', {
              'event_category': 'outbound',
              'event_label': url,
              'transport_type': 'beacon',
              'event_callback': function() {
                  document.location = url;
              }
          });
          return false;
      }
  </code></pre>
  
  <p>But I would like to set some additional parameters and according to <a href="https://developers.google.com/analytics/devguides/collection/gtagjs/events" rel="nofollow noreferrer">this</a> it should be possible:<br>
  "You can add custom data, in the form of additional parameters, to any event (recommended or custom)"</p>
  
  <p>I'm setting up GA using this: </p>
  
  <pre><code>    &lt;!-- Global site tag (gtag.js) - Google Analytics --&gt;
  &lt;script async src="https://www.googletagmanager.com/gtag/js?id=MY_GA_TRACKING_ID"&gt;&lt;/script&gt;
  &lt;script&gt;
      window.dataLayer = window.dataLayer || [];
  
      function gtag() {
          dataLayer.push(arguments);
      }
      gtag('js', new Date());
  
      gtag('config', 'MY_GA_TRACKING_ID');
  
  &lt;/script&gt;
  </code></pre>
  
  <p>And I'm calling <code>gtag()</code> the following way:</p>
  
  <pre><code>function trackOutboundLinkWithCustomParameters(label, version, url) {
          gtag('event', 'click', {
              'event_category': 'outbound',
              'event_label': label,
              'transport_type': 'beacon',
              'mobile_app_version': version,
              'download_url': url,
              'event_callback': function() {
                  document.location = url;
              }
          });
          return false;
      }
  </code></pre>
  
  <p>If I break at <code>dataLayer.push(arguments);</code> and prints <code>arguments</code> I get the following (which is what I want to track):
  <a href="https://i.stack.imgur.com/jUA0t.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jUA0t.png" alt="arguments"></a></p>
  
  <p>But after having installed <a href="https://chrome.google.com/webstore/detail/google-analytics-debugger/jnkmfdileelhofjcijamephohjechhna?hl=en" rel="nofollow noreferrer">GA Debugger</a> it seems it ignores my extra parameters when sending the tracking data:</p>
  
  <p><a href="https://i.stack.imgur.com/exY9C.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/exY9C.png" alt="GA Debugger output"></a></p>
  
  <p>Bottom line is: I can't see the additional info I provide about <code>mobile_app_version</code> and <code>download_url</code> in my GA dashboard. </p>
  
  <p>Help!</p>