I have tried to make a new custom webAPI route, but are this build error:

The type 'System.Net.Http.HttpMessageHandler' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Net.Http, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.

My custom route i put inside a file called "UmbracoEvents.cs" in the App_Start folder. And the content looks like:

using System.Web.Http;
using System.Web.Routing;
using Umbraco.Core;


namespace XXX
{
    public class UmbracoEvents : IApplicationEventHandler
    {
        public void OnApplicationInitialized(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
        }

        public void OnApplicationStarting(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
            RouteTable.Routes.MapHttpRoute(
                name: "DefaultApi",
                routeTemplate: "api/{controller}/{id}",
                defaults: new { id = RouteParameter.Optional }
            );

        }

        public void OnApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
        }

    }
}

Can anybody spot the hot to fix?