combining Register and Roles

// GET: /Account/Register
        [AllowAnonymous]
        public ActionResult Register()
        {
          
            ApplicationDbContext context = new ApplicationDbContext();
            var list = context.Roles.OrderBy(r => r.Name).ToList().Select(rr => new SelectListItem { Value = rr.Name.ToString(), Text = rr.Name }).ToList();
            ViewBag.Roles = list;
            return View();
        }

        //
        // POST: /Account/Register
        [HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public async Task Register(RegisterViewModel model,FormCollection form)
        {
            string roleName = form["RoleName"].ToString();
            var account = new AccountController();
          
            if (ModelState.IsValid)
            {
              
                var user = new ApplicationUser()
                {
                    UserName = model.UserName,
                    Email=model.Email,
                    FullName=model.FullName,
                    PhoneNumber=model.PhoneNumber,
                  

                };
               
                //ApplicationDbContext context = new ApplicationDbContext();
                //UserManager.AddToRole(model.Id, roleName);
               
                //ApplicationDbContext context = new ApplicationDbContext();

              
                var result = await UserManager.CreateAsync(user, model.Password);
                if (result.Succeeded)
                {
                    account.UserManager.AddToRole(user.Id, roleName);
                    await SignInAsync(user, isPersistent: false);
                    return RedirectToAction("Index", "Home");
                }
                else
                {
                    AddErrors(result);
                }
            }

            // If we got this far, something failed, redisplay form
            return View(model);
        }

You will have to Add some code in the //Account Controller /Register method

and then Add a dropdown list that containing the list of particular roles that you have created in the register View