HttpContext.Current.Response.AppendHeader("P3P", "CP=\"IDC DSP COR ADM DEVi TAIi PSA PSD IVAi IVDi CONi HIS OUR IND CNT\"");

public static void RegisterRoutes(RouteCollection routes)
{
  HttpContext.Current.Response.AppendHeader("P3P", "CP=\"IDC DSP COR ADM DEVi TAIi PSA PSD IVAi IVDi CONi HIS OUR IND CNT\"");
  routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
  routes.MapRoute(
      "Default", // Route name
      "{controller}/{action}/{id}", // URL with parameters
      new { controller = "Account", action = "Logon", id = UrlParameter.Optional }
  );

}

<system.webServer>
    <httpProtocol>
      <customHeaders>
        <add name="P3P" value='CP="CAO PSA OUR"'/>

public class P3PHeaderAttribute : ActionFilterAttribute
{
    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        filterContext.HttpContext.Response.AppendHeader("P3P", "CP=\"IDC DSP COR ADM DEVi TAIi PSA PSD IVAi IVDi CONi HIS OUR IND CNT\"");

    }
}

public static void RegisterGlobalFilters(GlobalFilterCollection filters)
    {
        filters.Add(new P3PHeaderAttribute());
    }