How to set Azure Authentication custom login return url?

I followed this article https://azure.microsoft.com/en-us/blog/announcing-app-service-authentication-authorization/ to set up Azure authentication for my MVC app. First I turned on Azure AD provider. In the Authentication / Authorization settings, I selected “Allow request(no Action)” for “Action to take when request is not authenticated” because I only need users to login for certain controller actions.

Is it possible to use async/await in MVC 4 AuthorizeAttribute?

The only override I see exposed on MVC’s AuthorizeAttribute is public override void OnAuthorization( AuthorizationContext filterContext ) which is not suitable for use with async/await because it doesn’t return a Task. Is there another way to create an AuthorizeAttribute in MVC that allows the use of async/await?