I actually taught one of my dogs long ago to do this. We screwed an eye screw into the door facing about a foot off of the floor. Then we attached a pet collar bell to the eye screw with a split ring. Each time we took the dog out (every hour or so) we'd take the dog's paw or nose and touch the bell enough to make it ring. Then we'd say, "You wanna go out?" Then open the door, take the pup out, let them go potty, then let them know how proud of them you are, back in you go. EVERY time, whether or not they meant to do it or if it was a mistake, that the bell rings, you take the dog out...NO MATTER WHAT! It doesn't matter if they ring it in the middle of a storm...they go out. They will learn that - ring the bell - "I go out". It works. The young dog I taught to do this...actually taught one of my older dogs to do it too!
Good luck!