We’ve had various discussions on this topic in the past in the Acoustics forum, so a search there should throw some threads into view.
Desk-mounted stands rarely sound as good as floor-mounted ones that tend to be more rigid, solid and heavier, and of course by mounting the speakers just above the desk you also run the risk of more reflections bouncing off it and into your ears that make your stereo imagery less focused.
However, essentially it comes down to how robust/rigid the desk is. If it’s rock solid and doesn’t ring at any frequencies then a solid stand would do the job reasonably well. Various stand manufacturers do lower stands at heights of 16“ and 12“, which may do the job for you (just make sure your tweeters end up pointing at your ears when sitting in your normal listening position, although there’s a little latitude either way since you can angle the loudspeakers slightly up or down to achieve this.
If on the other hand your desk isn’t a rock-solid monster, you’ll need decoupling between the stands and loudspeakers to prevent vibrations travelling in either direction (from speaker to desk, or from desk to speaker). This is where Mopads and the like are so useful.
Hope this helps!