How to get selected items from multiselect list and bind them in many to many relationship table

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C#

ASP.NET

MVC4

i have student table and Courses table the relationship between them is many to many relationship made by Student_Courses table.
i want to select subjects which the student will study when register a new student

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here is the courses class in the model.

public partial class TblCours
    {
        public TblCours()
        {
            this.TblStudents = new HashSet<TblStudent>();
        }

        public int CourseId { get; set; }
        public string CourseName { get; set; }

        public virtual ICollection<TblStudent> TblStudents { get; set; }
    }

and this is the student class in the model

public partial class TblStudent
    {
        public TblStudent()
        {
            this.TblCourses = new HashSet<TblCours>();
        }

        public int StudentId { get; set; }
        public string StudentName { get; set; }
        public Nullable<short> StudentAge { get; set; }

        public virtual ICollection<TblCours> TblCourses { get; set; }
    }

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this is the createStudent View

@{
    ViewBag.Title = "Create";
    SelectList items = ViewBag.subjectsList;
}
@model School.Models.TblStudent


<h2>Create</h2>
@using (Html.BeginForm())
{
    @Html.TextBoxFor(model => model.StudentId, new {  @class = "form-control" })
    @Html.TextBoxFor(model => model.StudentName, new {  @class = "form-control" })
    
     @Html.ListBoxFor(model => model.TblCourses, items as MultiSelectList);
    
    <input type="submit" class="btn btn-danger btn-lg center-block" onclick="validedStyle()" value="Add student">
     
}

and this is the controller actions

public IEnumerable<TblCours> GetCourses()
       {
           List<TblCours> cntTable = model.TblCourses.ToList();
           return cntTable;
       }

       //
       // POST: /Student/Create

       [HttpPost]
       public ActionResult Create( TblStudent std)
       {
           try
           {
               model.TblStudents.Add(std);
               model.SaveChanges();
               ViewBag.subjectsList = new SelectList(GetCourses(), "CourseId", "CourseName");
               ViewBag.selection = GetCourses();
               return View();
           }
           catch
           {
               return View();
           }
       }

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