This is an old “unanswered” post so I figured I would add my two cents. Adding request headers might not work in general, depending on how the ARR module/handler will process the request. It may be too late as Tarun mentioned, but for another reason. ARR module may be grabbing the raw data being received, in which case it definitely will be too late. Or ARR handler may be looking at particular Server Variables containing the original headers which were set before your notification (e.g. I don’t think ALL_RAW is updated if you add additional request headers.) In the likely case ARR module is grabbing raw buffers, you would need to get ahead of that and update the raw data, which, if possible, is a whole new level of complexity.