MVC-如何获取ListBox所选项目'；s在控制器中的值

我有一个应用程序，它有两个列表框：

 @using (Html.BeginForm())
        {
           @Html.ListBoxFor(m => m.SelectedAttributes, Model.Attributes, new {id="listBoxAvail", SIZE = 5} ) 
            <button type="button" id="add">MoveRight</button>
            <button type="button" id="remove">"MoveLeft"></button>
            <button type="button" id="remove-all">RemAll</button>
            <button type="button" id="select-all">SelAll</button>
            <button type="button" id="up" onclick="MoveUp()">Up</button>
            <button type="button" id="down" onclick="MoveDown()">Down</button>
            @Html.ListBoxFor(m => m.SelectedAttributes2, Model.SelectedItems, new { id = "listBoxSel", SIZE = 5})
            <br />
            <p>Item Caption Text (140 Characters. HTML not allowed.)</p>
            @Html.TextAreaFor(m => m.ItemCaptionText)
            <input type="submit" id="submit"  value="Save"/>
        } 

我使用jQuery将选定的值从一个列表框转移到另一个列表盒，但我在值后面加上一个"L"以供以后使用：

$("#add").click(function () {
                $("#listBoxAvail > option:selected").each(function () {
                    $(this).remove();
                    val = $(this).val();
                    temp = $(this);
                    temp.val('L' + val)
                    $(temp).appendTo("#listBoxSel");
                });
            });

使用F12开发人员工具转移到右侧列表框后，我会看到文本和值。我想知道的是如何从控制器中的C#代码中获得该值？

[HttpPost]
        public ActionResult Index(OptInViewModel viewModel)
        {
            AttributeEntities db = new AttributeEntities();
            IEnumerable<string> items = viewModel.SelectedAttributes2;
            foreach (var item in items)
            {
                var temp = item;
                // Save it
                SelectedHarmonyAttribute attribute = new SelectedHarmonyAttribute();
                attribute.CustomLabel = viewModel.ItemCaptionText;
                //attribute.IsVisible = ?
                //attribute.OrderNumber = ?
                //attribute.HarmonyAttribute_ID
            }
            return View("Index");
        }

ViewModel是：

public class OptInViewModel
    {
        public IEnumerable<string> SelectedAttributes { get; set; }
        public IEnumerable<string> SelectedAttributes2 { get; set; }
        public IEnumerable<SelectListItem> Attributes { get; set; }
        public IEnumerable<SelectListItem> SelectedItems { get; set; }
        public string ItemCaptionText { get; set; }
    }