Backend.Controllers.AuthenticationController.SignIn C# (CSharp) Method

        public ActionResult SignIn(string returnUrl = null) {
            // Note: the "returnUrl" parameter corresponds to the endpoint the user agent
            // will be redirected to after a successful authentication and not
            // the redirect_uri of the requesting client application.
            ViewBag.ReturnUrl = returnUrl;

            // Note: in a real world application, you'd probably prefer creating a specific view model.
            return View("SignIn", HttpContext.GetExternalProviders());
        }