<div class="col-lg-6">
@{using (Html.BeginForm("Results", "Searchv2", FormMethod.Get)) {
  <div class="input-group">
    <div class="input-group-btn">
      <button type="button" class="btn btn-default dropdown-toggle" data-toggle="dropdown">Action <span class="caret"></span></button>
      <ul class="dropdown-menu">
      @Html.DropDownList("Searchby", new SelectList(new[] { "User", "Restaurant", "Cuisine" }))
      </ul>
    </div>
    <input type="text" class="form-control" placeholder="Search" name="SearchString" id="SearchString">
  </div>
}}
</div>

<ul class="dropdown-menu">
@foreach (var searchType in new[] { "User", "Restaurant", "Cuisine" })
{
    <li><a href="#">@searchType</a></li>
}
</ul>

<input type="hidden" name="SearchBy" />
<script>
    $(".dropdown-menu a").click(function() {
        $("input[name='SearchBy']").val($(this).html());
    });
</script>