There are many ways to mount a hub under your desk.
Does the hub have any key-way style screw holes in the bottom of the case?
If so, you can use a couple of pan head screws and slide the hub over the screw heads.
If you plan this as a permanent installation, double stick foam tape works very well.
Along the same lines, you can get various style cable tie mounts that either stick or screw to your desk, and then use cable ties to secure the hub to the mounts.
You can also get cable ties with a boss in the end of them securing the cable tie to a surface with a screw.
Do you have another use for the hub?
If so, consider installing a hub that mounts in a spare drive bay.
Or, install a 4 port USB 2.0 card.
Chas