Pass id value selected in dropdowlist to the controller to filter webgrid

I had previously asked a question that thanks to Davlio has already been resolved.

However, another question arose, I have two stored procedures mapped in entityframework, one to fill dropdownlist (GetDepartamentos_Result) and another to fill webgrid depending on the selection you make in the dropdownlist (SearchEmployeesDropDown_Result).

The question is that from what I have read you can not have two models in a view so I made a new model that my two models of my stored procedure contain, leaving as follows:

   namespace WebgridPagingSortingFiltering.Models
     {
          public class ViewModels
             {
               public List<GetDepartamentos_Result> Combo { get; set; }
                 public List<SearchEmployeesDropDown_Result> Grid { get; set; }
                  public int SelectedDeparmentID { get; set; }
              }
         }

add a new property SelectedDeparmentID for the value of the Id of dropdownlist .

In my controller I have the following two methods one to fill the dropdownlist and another one to populate the wegrid :

To fill drop :

  public List<GetDepartamentos_Result> GetDepartamentos()
        {
            using (TigerEntities db = new TigerEntities())
           {
               ViewModels vm = new ViewModels();
               vm.Combo = db.GetDepartamentos().ToList();
               return vm.Combo;
           }
        }

To fill webgrid :

 public List<SearchEmployeesDropDown_Result> GetEmpleadosbyDrop (int Id_dep)
       {
           using (TigerEntities db = new TigerEntities())
          {
               ViewModels vm = new ViewModels();
               vm.Grid = db.SearchEmployeesDropDown(Id_dep).ToList();
               return vm.Grid;
           }
        }

And I have an actionresult method that loads the data in the view for dropdownlist :

 public ActionResult Combo()
       {
        var a = GetDepartamentos();
        ViewBag.Departamentos = new SelectList(a, "Id_Departamento", 
        "Nombre_Departamento");
        return View(b);
       }

In my view I have the following:

   @model WebgridPagingSortingFiltering.Models.ViewModels

   @{
      Layout = null;
      var grid = new WebGrid(canPage: true, rowsPerPage: 4);
      grid.Bind(source: Model.Grid, rowCount: ViewBag.TotalRows);
    }

    @using (Html.BeginForm("combo", "home", FormMethod.Post, new { @class = 
    "navbar-form navbar-left" }))
                {
                    <div class="form-group">
                        @Html.DropDownListFor(model => 
      model.SelectedDeparmentID, ViewBag.Departamentos as SelectList, new { 
     @class = "form-control" })
                    </div>
                    <button type="submit" value="Search" class="btn btn-
     success">Search</button>
                }

My question is how I pass the value of the dropdownlist's id to the controller so that it can filter with the stored procedure I have done and show me the data in the webgrid.

I have the same actionresult method but I do not know what to do as a parameter:

 [HttpPost]
       public ActionResult Combo(??) No se si pasarle el modelo ViewModels o 
     el SelectedDeparmentID
     {
        var a = GetDepartamentos();
        ViewBag.Departamentos = new SelectList(a, "Id_Departamento", 
            "Nombre_Departamento");
        var b = GetEmpleadosbyDrop(??);
        return View(b);
      }

I would greatly appreciate your help that you can provide me. Greetings