The person who gets promoted is usually the one who was already doing half of the next job.

Promotions usually confirm something that has already happened rather than starting it. Whoever hands them out is looking for evidence, and the cheapest evidence available is somebody who has quietly been handling pieces of the bigger role already. From outside it looks like a reward. From inside it is mostly paperwork catching up with the facts.

Find the part of the job above you that nobody enjoys and start doing it before anyone asks. Open up, train the new person, chase whatever keeps getting dropped. Then when you ask, you are describing what you already do rather than what you intend to do.