[HttpGet]
        public IActionResult Login(string returnUrl = null)
        {
            return View(new LoginModel { ReturnUrl = returnUrl });
        }

[HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Login(LoginModel model)
        {
            if (ModelState.IsValid)
            {
                User user = await db_context.User
                    .Include(u => u.Role)
                    .FirstOrDefaultAsync(u => u.UserName == model.UserName && u.Password == model.Password);
                if (user != null)
                {
                    await Authenticate(user); // аутентификация

                    // проверяем, принадлежит ли URL приложению
                    if (!string.IsNullOrEmpty(model.ReturnUrl) && Url.IsLocalUrl(model.ReturnUrl))
                    {
                        return Redirect(model.ReturnUrl);
                    }
                    else
                    {
                        return RedirectToAction("Index", "Friends");
                    }
                }
                else
                {
                    ModelState.AddModelError("", "Неправильный логин и (или) пароль");
                }
            }
            return View(model);
        }

<div class="container">
    <div class="row">
        <div class="col-md-offset-3 col-md-6"></div>      
        <div class="col-sm-4">
            <span class="heading" style="align-items:center">АВТОРИЗАЦИЯ</span>
            <form class="form-horizontal" asp-controller="Account" asp-action="Login" method="post">
                <div class="form-group">
                    <label class="control-label">Имя пользователя</label>
                    <input asp-for="UserName" class="form-control" type="email" id="inputEmail" placeholder="E-mail" />
                    <span asp-validation-for="UserName"></span>
                    <i class="fa fa-user"></i>
                </div>
                <div class="form-group">
                    <label class="control-label">Пароль</label>
                    <input asp-for="Password" class="form-control" type="password" id="inputPassword" placeholder="Password" />
                    <span asp-validation-for="Password"></span>
                    <i class="fa fa-lock"></i>
                    <a href="#" class="fa fa-question-circle"></a>
                </div>
                <div class="form-group">
                    <input type="submit" class="btn btn-default" value="Войти"/>
                </div>
                <div class="form-group">
                    <li class="nav-item">
                        <a class="nav-link text-dark" asp-area="" asp-action="Register" asp-controller="Account">
                            <font size="2" color="blue" face="Arial">Зарегистрироваться</font>
                        </a>
                    </li>
                </div>
            </form>
        </div>
        <div class="col-sm-4"></div>
    </div>
</div>

asp-route-returnUrl="@Model.ReturnUrl"