I want to populate data from dropdownlist to text box

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MVC

I have an Employee module and a Manager module.
In the employee registration page A drop down list is there where one can select a manager present in the database. So if u select the drop down list then that manager name should print in the text box.
My Module is

public partial class AttendanceEntities : DbContext
  {
      public AttendanceEntities()
          : base("name=AttendanceEntities")
      {
      }

      protected override void OnModelCreating(DbModelBuilder modelBuilder)
      {
          throw new UnintentionalCodeFirstException();
      }

      public virtual DbSet<Administrator> Administrators { get; set; }
      public virtual DbSet<Attendance_Code> Attendance_Code { get; set; }
      public virtual DbSet<Attendance_Log> Attendance_Log { get; set; }
      public virtual DbSet<Employee> Employees { get; set; }
      public virtual DbSet<Manager> Managers { get; set; }

my controller is

ViewBag.ManagerId = new SelectList(db.Managers, "ManagerId", "UserName", e.ManagerName);

my view is

<div class="col-md-10">
              @Html.DropDownList("ManagerId", "Select Manager")

          </div>

What I have tried:

I have tried some code but Manager Id is coming instead of Manager Name