ASP.NET Web API Route Controller Not Found

I am trying to post to the following Web API:

http://localhost:8543/api/login/authenticate

LoginApi (Web API) is defined below:

[RoutePrefix("login")]
public class LoginApi : ApiController
{
    [HttpPost]
    [Route("authenticate")]
    public string Authenticate(LoginViewModel loginViewModel)
    {  
        return "Hello World"; 
    }
}

WebApiConfig.cs:

public static void Register(HttpConfiguration config)
{
    // Web API configuration and services

    // Web API routes
    config.MapHttpAttributeRoutes();

    config.Routes.MapHttpRoute(
        name: "DefaultApi",
        routeTemplate: "api/{controller}/{id}",
        defaults: new { id = RouteParameter.Optional }
    );
}

Here is the error I get:

Request URL:http://localhost:8543/api/login/authenticate
Request Method:POST
Status Code:404 Not Found
Remote Address:[::1]:8543