During career mode you will have to do the FMX training program. After completing this go to the garage and customize your rider by buying all 3 Excavator items ( you can buy the cheapest set if you want to save money )
Go to the Urban sprawl event and pick the track Daredevil Showdown, right at the start there`s a small jump with a ring of fire around it, go slowly off the edge and fall in between the buildings and while keeping your bike level/straight hold DOWN on the right analogue stick to perform the proud hero pose and as soon as you crash the trophy will pop...
Here`s a video guide aswell (CREDIT TO IGN FOR THE VIDEO)