I found with Kodi that it happened by itself, but it took time. From the very beginning, Kodi would go when he he needed to if he was outdoors. He learned early that both outdoors and his litter boxes were "OK" places to potty... everywhere else was "not OK".
Very early, he showed a definite preference for pooping outdoors, even though he was happy to pee in the litter box. He would only poop in the litter box if there was no other option.
But I have to say, that for almost his first 2 years, while he MOSTLY went outdoors, because I took him out at the times I knew he'd need to go, he NEVER "asked" to go out. I know some people are successful teaching their dog to use bells as a signal, but although Kodi is very bright, and very trainable in other ways, he NEVER caught onto the bells, even after months of consistent work. I finally took them down because they were scratching the woodwork and he never used them independently.
Then, shortly after his second birthday, I heard this deep "woof", a sound I'd never heard him make before, and went to find him standing at the door. We went out, he pottied, and came right back in. From that day, this deep "woof" became his signal, one he made for himself. I know other dogs use other signals.
I know Tom King has said many times that for most pups, good potty behavior is "habit" (and the responsibility of the owner) for at least the first year, often for much longer. It is only as they mature that, eventually, they make the cognitive connection between the "habit" and "requesting" to go out.
When you think about it, to start with, pottying is like a binary switch for a long time... the puppy learns "OK" and "not OK" places. That's MUCH easier to learn/teach than if/then scenarios. So you will probably have a LONG time when he will know that it's OK to potty on his pad OR outside, before he makes the connection that IF he gives you a signal, THEN you will take him out. It will take even longer for him to learn (if he ever really does) that IF you are home, he should tell you he needs to go out, and only IF you aren't there or can't take him out, THEN he should use the pad. See how much more complicated that is? Especially without a common language?
You may get to the stage that Kodi is, where he only uses the litter box when we can't take him out, but I don't for one minute believe that Kodi understands that this is what WE want. He just knows that if he can't get us to take him out (which he would prefer) the litter box is the other "OK" potty option.
See the difference? The result may be the same, but the understanding and intent from the dog's point of view is very different.