Adding a dropdown of Departments in the in Register.cshtml and retrieving its selected value in the Reginster action method

In my Asp:Net MVC project I want to add departments to the Registerform. When I try to register a user I get this error

There is no ViewData item of type 'IEnumerable' that has the key 'DepartmentId'.

and I think it's something to do with my Register action. This is my Department class in my Model

public class Department
  {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int DepartmentId { get; set; }
        [Required]
        [MaxLength(50)]
        public string DepartmentName { get; set; }
    }

Then I added this two properity to RegisterViewModel

public int DepartmentId { get; set; }
    public Department Department { get; set; }

Looks like this below..

public class RegisterViewModel
    {
        [Required]
        [EmailAddress]
        [Display(Name = "Email")]
        public string Email { get; set; }


        [Required]
        public string FirstName { get; set; }


        [ForeignKey("Department")]
        public int DepartmentId { get; set; }
        public Department Department { get; set; }

        [Required]
        [StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
        [DataType(DataType.Password)]
        [Display(Name = "Password")]
        public string Password { get; set; }

        [DataType(DataType.Password)]
        [Display(Name = "Confirm password")]
        [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
        public string ConfirmPassword { get; set; }
    }

Then In my Register action I dont know how to code it .. tried many kind of codes but don't know how to do it.

// Probably it's here the very problem , i think.

// GET: /Account/Register
        [AllowAnonymous]
        public ActionResult Register()
        {
           // Here inside.. I don't know how to do... ;)
            ViewBag.DepartmentId = new SelectList(?????, "DepartmentId", "DepartmentName");
        Or maybe:
            ViewBag.DepartmentId = new IEnumerable<SelectListItem> .....;


            return View();
        }

        // And this is what my Register looks like
        // POST: /Account/Register
        [HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public async Task<ActionResult> Register(RegisterViewModel model)
        {
            if (ModelState.IsValid)
            {
                var user = new ApplicationUser {FirstName = model.FirstName, DepartmentId = model.DepartmentId, Email = model.Email };
                var result = await UserManager.CreateAsync(user, model.Password);
                if (result.Succeeded)
                {
......

And RegisterView


    @model PersonRegister.Models.RegisterViewModel
        ....
        <div class="form-group">
                @Html.LabelFor(m => m.DepartmentId, new { @class = "col-md-2 control-label" })
                <div class="col-md-10">
                    @Html.DropDownList("DepartmentId", null, htmlAttributes: new { @class = "form-control" })
                </div>
            </div>

.....
... and here rest of view such as... email.....

Thank you in advance!

Example:

  [AllowAnonymous]
    public ActionResult Register()
    {

        ViewBag.DeptList = _context.Departments
                                  .select(s=> new SelectListItem
                                  {
                                   Value=s.DepartmentId.ToString(),
                                   Text=s.DepartmentName
                                  }).ToList();//select data from determent table 

        return View();
    }

View

  @model PersonRegister.Models.RegisterViewModel
        ....
        <div class="form-group">
                @Html.LabelFor(m => m.DepartmentId, new { @class = "col-md-2 control-label" })
                <div class="col-md-10">
                 @Html.DropDownListFor(m => m.DepartmentId , new SelectList((IEnumerable)ViewBag.DeptList,"Value","Text",Model.DepartmentId ), new { @class = "form-control" })
                </div>
            </div>

OR

public class RegisterViewModel
    {
         Required]
    [EmailAddress]
    [Display(Name = "Email")]
    public string Email { get; set; }


    [Required]
    public string FirstName { get; set; }

    public int? DepartmentId { get; set; }   

    [Required]
    [StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
    [DataType(DataType.Password)]
    [Display(Name = "Password")]
    public string Password { get; set; }

    [DataType(DataType.Password)]
    [Display(Name = "Confirm password")]
    [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
    public string ConfirmPassword { get; set; }
        [Ignore]
        public List<SelectListItem> DepartmentList {get;set;}
    }

  [AllowAnonymous]
    public ActionResult Register()
    {
      var model= new RegisterViewModel();
        model.DepartmentList = _context.Departments
                                  .select(s=> new SelectListItem
                                  {
                                   Value=s.DepartmentId.ToString(),
                                   Text=s.DepartmentName
                                  }).ToList();//select data from determent table 

        return View(model);
    }

View

  @model PersonRegister.Models.RegisterViewModel
        ....
        <div class="form-group">
                @Html.LabelFor(m => m.DepartmentId, new { @class = "col-md-2 control-label" })
                <div class="col-md-10">
                 @Html.DropDownListFor(m => m.DepartmentId ,Model.DepartmentList,"--Select--" ,new { @class = "form-control" })
                </div>
            </div>