Capture Textbox value through a CheckBox asp.net MVC

There is a Form which consists of Textbox which are activated or deactivated by a CheckBox ....

How can I capture the value that the user enters validating that the Check is marked?

View

My View:

  <script type="text/javascript">
        $(function (){
            $("#idcheckproveedor").change(function(){
                var st = this.checked;
                if (st) {
                    $("#txtSearch").prop("disabled", false);
                }
                else {
                    $("#txtSearch").prop("disabled", true);
                }
                });
            });
    </script>

  <div class="form-group">
  <label>Proveedor</label>
  <input type="checkbox" id="idcheckproveedor" checked="checked" />
  @Html.TextBox("searchTerm", null, new { @class = "form-control", id = "txtSearch" })
 </div>

My Controller:

 [HttpPost]
        public ActionResult BuscarRecepcion(string searchTerm, int? page, string fi, string ff, int NumeroFactura)
        {          

            if (string.IsNullOrEmpty(searchTerm))
            {
                var abrir = db.Entradas.ToList();
                ViewBag.Error = "Debe Ingresar un Proveedor";
                return View(abrir.ToList().ToPagedList(page ?? 1, 60));
            }           


                //CONSULTO ID PROVEEDOR
                var buscarproveedor = db.Proveedors.Where(n => n.v_Nombre == searchTerm).Select(c => c.Kn_CodigoProveedor).FirstOrDefault();

                var nombreproveedor = db.Proveedors.Where(n => n.Kn_CodigoProveedor == buscarproveedor).Select(b => b.v_Nombre).FirstOrDefault();

                if (nombreproveedor == null)
                {
                    ViewBag.Error = "Debe Seleccionar un Producto o Material Válido";
                    return View();
                }               

                return View(nombreproveedor.ToList().ToPagedList(page ?? 1, 60));
            }