<form action=“{{ site.baseurl }}/{{ include.slug }}” method=“get”>

<div class="input-group">
  <input type="text" id="search-box" name="s" placeholder="{{ site.search.placeholder | default: include.placeholder_fallback }}" class="form-control" value="">
  <input name="ga_category" value="{{ site.search.ga_category }}" type="hidden">
  <span class="input-group-btn">
    <button type="submit" class="btn"><i class="fa fa-search fa-fw"></i></button>
  </span>
</div>

</form>