If you don't finish...

... your meal, you will not get any dessert!

So what is wrong with this statement?
Everything!
Dessert is used as a reward, which means that getting no desert is a punishment. Useless & respectless.
Why force a child to eat more than the child feels comfortable with in the first place, that in itself can breed obesity, which I doubt any parent wants that.
Besides, what's the beef anyway?
Why not take dessert for starters?
Why not make dessert the most important meal?
If you make sure dessert is about as healthy as the meal there is no issue with it at all.
I refuse to use that. My children eat from each course what they want, sometimes they skip dessert as they prefer the starter or the main course.
We don't need that type of stress at our dining table.

Sceptical? Why not try it for a few weeks and see if they still feel dessert is the most important?

That actually only happens when its used as a reward, then the focus goes to the dessert, not the rest, it all of a sudden makes the rest of the meal totally uninteresting, while when there is no stress about it, the whole meal is interesting. Really, just give it a try.

Have fun!