My oldest doesn't ask to go out and never really has. We just got into a routine, especially on week days. Like we let him out before we start eating and again when we have a brew. Generally he will stand at the door when he wants to go out but when we're not in the room this doesn't get him anywhere.
You can teach them to ring a bell for example when they want to go out but Ive never tried it. Ive read that you put a pile of biscuts outside. Let the dog play with the bell and encourgage any sort of interaction. When he accidentally rings the bell. Open the door and let him get the prize. Then do a few repetitions of this. Once hes got the hang of it stop leaving biscits outside and only reward after hes weed. I don't know how successful this method is as i haven't personally tried it
Once he makes the connection between the bell and the door being opened to get let out he'll start to work out how to ask. But you'll find he'll having you opening and closing the door lots in the early days while its still a novelty and while hes still hoping that pile of biscits will reappear
