Yes willpower is too simple and too tied up with connotations of hardworkingness and virtue to adequately explain it. It's more like "don't have control over where attention is focused." Wanting to do it may make it more likely but there are other factors, and I can also easily get caught up in a task that is actively unpleasant. Unmedicated the focus goes where it wants to go, not where I want it to go; but not necessarily nowhere either.

And ADHD people in my experience aren't worse at activities that do require what I would call simple willpower. Things like race cycling or endurance running, where persisting through discomfort and even pain are necessary.