boxes/_includes/analytics.html

{% if site.gaaccount != '' %}
<script async src="https://www.googletagmanager.com/gtag/js?id={{ site.gaaccount }}"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());
  gtag('config', '{{ site.gaaccount }}', {'anonymize_ip': true});
</script>
{% endif %}