codeninjasuk.github.io/_includes/search/google-search-scripts.html

<script>
  (function () {
    var cx = '{{ site.google.search_engine_id }}';
    var gcse = document.createElement('script');
    gcse.type = 'text/javascript';
    gcse.async = true;
    gcse.src = 'https://cse.google.com/cse.js?cx=' + cx;
    var s = document.getElementsByTagName('script')[0];
    s.parentNode.insertBefore(gcse, s);
  })();

  function googleCustomSearchExecute() {
    var input = document.getElementById('cse-search-input-box-id');
    var element = google.search.cse.element.getElement('searchresults-only0');
    if (input.value == '') {
      element.clearAllResults();
    } else {
      element.execute(input.value);
    }
    return false;
  }

  {% if site.google.instant_search %}
    $(document).ready(function () {
      $('input#cse-search-input-box-id').on('keyup', function () {
        googleCustomSearchExecute();
      });
    });
  {% endif %}
</script>