I think cycling is the wrong motion for an under-desk exercise machine. I think it should be a very short vertical motion, just feet moving up-and-down, something you can do without disrupting your chair posture, or making the rest of your body move too much.
Potentially you don't even lift your toes up, just raise and lower your heels with some kind of resistance.
Maybe you wouldn't get much exercise this way, but I think getting a whole lot of exercise whilst trying to work at a computer just isn't really doable.
I wonder if even that might be too much motion for under a desk. Plus, it looks like it's designed to be stood on, so I wonder how it would work when you're sitting and not directly over it, and don't have all your weight on it. Still, might be an options.
I've also thought a sliding back-and-forth motion might work -more like a skiing thing.
Partially kidding, partially serious. Careful: These aren't noiseless. My former downstairs neighbor wasn't too happy with me so I could only practice when he wasn't at home ;-)
Potentially you don't even lift your toes up, just raise and lower your heels with some kind of resistance.
Maybe you wouldn't get much exercise this way, but I think getting a whole lot of exercise whilst trying to work at a computer just isn't really doable.