<div class="input-group b-r-xl">
  @Html.EditorFor(model => model.Buscar, new { htmlAttributes = new { @class = "form-control", placeholder = "Busqueda por asunto, numero o fecha", value = "
  <?=$_POST['var']?>" } })
    <div class="input-group-btn">
      <input type="submit" value="Buscar" name="model.Buscar" class="btn btn-primary" />
    </div>
</div>