@using (Html.BeginForm())
{
@Html.TextBox("searchString", (string)ViewBag.searchString, new { @class = "form-control", name = "searchString", placeholder = "Search..." })
<input type="submit" value="Search" class="btn" />
@Html.ActionLink("Search", "Index", new { sortOrder = @ViewBag.sortOrder, searchString = ???,  page = 1 }, new { @class = "btn", type = "submit" })
}