User.Identity.IsAuthenticated.cs

//Azure AD B2C Owin Context is null , once the token is acquired by authorization code flow
//Set Authentication Manager profile

var authenticated = HttpContext.GetOwinContext().Authentication.AuthenticateAsync("ExternalCookie");
// Send Access Token bearer to api to get logged user details 

var loggedInUser = AuthenticationManager.BuildVirtualUser(string.Format(@"{0}\{1}", "external", "User-ID"), true);
loggedInUser.RuntimeSettings.AddedRoles.Add(authenticated.Result.Identity.Claims.First().Value);
loggedInUser.Profile.FullName = string.Format("{0} {1}", authenticated.Result.Identity.Name, authenticated.Result.Identity.Claims.Last().Value);
loggedInUser.Profile.Save();
AuthenticationManager.Login(loggedInUser);
userObject.IsAuthenticated = User.Identity.IsAuthenticated;