trick question, there are many ways to get the AVR under your desk. How permanent is the installation? How much money do you want to spend? What is your proposed budget? What kind of desk? Got pics? What AVR, how much does it weigh?
No matter how you mount it, make sure it has room above it and to the sides for airflow, and a place to actually vent the warm air. That might mean taking a backing panel off of the desk and adding a computer fan or two.
The easiest solution is to put a small table or stand under the desk, the down side is it would eat up leg room.
If you could also build a small shelf/bracket that would screw into the bottom of the desktop.