Fetching and Adding Checkbox dynamically on selection of Dropdown boxes

I have a form as above. I am trying to work as when user selects Table Name & Permission, it goes back to server side, fetches the columns of the selected table & display all column names as check boxes. When use selects Save btn, HttpPost will execute and when user selects Cancel, to return back to home page.

I have created a ViewModel for this :

 // Actual EF Model
    public partial class TablePermission
{
    public int Id { get; set; }
    public int UserId { get; set; }
    public int PermissionLevelId { get; set; }
    public string TableName { get; set; }
    public string RestrictViewFields { get; set; }
    public string RestrictEditFields { get; set; }

    public virtual PermissionLevel PermissionLevel { get; set; }
    public virtual User User { get; set; }
}
// View Model for the View
public class TablePermissionsVM
{

    public TablePermissionsVM()
    {
        TablePermission = new TablePermission();
        RestrictViewFields = new List<FieldList>();

        // Created for trial to see Checkboxes 
        RestrictViewFields.Add(new FieldList() { FieldName = "UserId", Selected = false });
        RestrictViewFields.Add(new FieldList() { FieldName = "fName", Selected = false });
        RestrictViewFields.Add(new FieldList() { FieldName = "lName", Selected = false });

        RestrictEditFields = new List<FieldList>();
    }

    public TablePermission TablePermission { get; set; }

    public List<FieldList> RestrictViewFields { get; set; }

    public IEnumerable<FieldList> RestrictEditFields { get; set; }
}

// Model to save field names & it's selected status
public class FieldList
{
    public string FieldName { get; set;  }
    public bool Selected { get; set; }
}
}

Controller UPDATED : ADDED THE NEW ACTION (FillFields() ) METHOD that has to called onChange event

       [Authorize]
    [HttpGet]
    public ActionResult TablePermissions(TablePermissionsVM tablePermissionVm)
    {
        return View(tablePermissionVm);
    }

    // Action Method to Fill Column names for the List<>.
    public ActionResult FillFields(string tableName, string tblPermLevel)
    {
        // WANT TO RETURN HERE ANOTHER LIST (2 LIST OBJECTS) IN JSON
        // restrictView & restrictEdit 
        var restrictView = DbUtilities.GetColumnNames(tableName);
        var restrictEdit = DbUtilities.GetColumnNames(tableName);
        return Json(restrictView, JsonRequestBehavior.AllowGet);
    }

View - UPDATED CODE : aDDED Bound fields for TableName & TableLevelPermission, Script that I use on the event of change of Table Selected. UPDATED - aDDED FORM ID, SCRIPT METHOD

       @model DataStudio.Models.TablePermissionsVM

using (Html.BeginForm("TablePermissions", "Admin", FormMethod.Post, new { id = "tblPermForm" }) ))
{
    @Html.AntiForgeryToken()

        <div class="form-group">
            @Html.LabelFor(model => model.TablePermission.TableName, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="editor-field">
                @Html.DropDownListFor(model => model.TablePermission.TableName, 
                    DbUtilities.GetTableNames(), "Select Table", 
                    new { @class = "form-control", @onchange="FillFields()" })
                @Html.ValidationMessageFor(model => model.TablePermission.TableName, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            @Html.LabelFor(model => model.TablePermission.PermissionLevelId, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="editor-field">
                @Html.DropDownListFor(model => model.TablePermission.PermissionLevelId, DbUtilities.GetPermissionLevelList(), "Select Permission Level", new { @class = "form-control" })
                @Html.ValidationMessageFor(model => model.TablePermission.PermissionLevelId, "", new { @class = "text-danger" })
            </div>
        </div>

       <div class="form-group">
            @Html.LabelFor(model => model.RestrictViewFields, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="editor-field">
                 **// NEED TO ADD CHECK BOXES HER RECEIVED THRU SCRIPT** 
            </div>
        </div>
  }

<script>
    function FillFields() {
        var tblName = $('#TablePermission_TableName').val();
        var tblPermLevel = $('#TablePermission_PermissionLevelId').val();

        //($('tblPermForm').valid()) { ERROR - OBJECT DOESN'T HAVE valid()'
        if (tblName != null && tblPermLevel != null) {
            $.ajax({
                url: '/Admin/FillFields',
                type: 'GET',
                dataType: "JSON",
                data: { TableName: tblName, TablePermLevel: tblPermLevel },
                success: function (restrictView) {
                    $("#RestrictViewFields").html("");  // Clear before appending new ones
                    $.each(restrictView, function (i, field) {
                        $("#RestrictViewFields").append(
                            $('<option></option>').val(field.FieldName).html(field.FieldName))  
                  // WANT TO ADD AS 3 CHECKBOX IN A ROW
                    });

                }
            });
        }
    }
  </script>

Their are couple of things that I am not able to figure out & get confused with it. Mainly, on making sure that both the drop down boxes has value, I need to perform again a "Get" and fetch column names for the table selected & display the columns as check boxes. The way I have implemented Checkboxes, I will get proper selected Values in HttpPost, Right ! Are am I anywhere wrong ?

How to make the Get Request when both the drop down's are selected ??

Any help is highly appreciated. Thanks a lot in advance.

Update I started to try with only TableName selection (I want for both dropdown), but the event doesn't occur and go to FillFields() in script. Where am I going wrong ? I tried this logic from here . Can't get why it doesn't get fired only ??? Btw, this is a full form i mean, their is no partial form in it. I want to fill the check box controls in those 2 RestrictXFields on selection of TableName & Permssion check box & on Save btn, send all to Request & save to db.

UPDATE : THANKS a lot, Stephen & Chethan. With your support, I identified the cause for event not getting triggered. Event is triggered, I am able to retrieve the column names from db, the HTML part on success is not being updated. Stephen, I also added form Id & tried form.valid() as you instructed, but I get error script doesn't identify valid(). Both the fields in Model are marked as Required in MetaData class. Currently, testing both var != null works. But, I liked you valid() option.