@Html.DropDownList(model => model.Dept_ID, ViewBag.deptlistname as
SelectList, "Please select a department");

public ActionResult UserRegistration()
{
     HREntities4 db = new HREntities4();
     var getdeptlist = db.departments.ToList();
     SelectList list = new SelectList(getdeptlist, "Dept_ID",
     "Dept_ID");
     ViewBag.deptlistname = list;


     return View();
 }

[HttpPost]
public ActionResult UserRegistration(UserRegistration model)
{
    if (ModelState.IsValid)
    {
    var details = new HREntities4();
    details.logins.Add(new login{
       FirstName = model.FirstName,
       LastName = model.LastName,
       Username = model.Username,
       Email = model.Email,
       Password = model.Password,
       PhoneNumber =model.PhoneNumber,
       Address = model.Address,
       Hire_Date = model.Hire_Date,
       Salary = model.Salary,
       Dept_ID = model.Dept_ID
    });
    details.SaveChanges();
    ModelState.AddModelError("updated", "User has been registered");
        }
    return View();

    }

public partial class UserRegistration
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Username { get; set; }
    public string Email { get; set; }
    public string Password { get; set; }
    public string PhoneNumber { get; set; }
    public string Address { get; set; }
    public string Hire_Date { get; set; }
    public Nullable<double> Salary { get; set; }
    public int Dept_ID { get; set; }

}