Using a suction cup style dent puller on plastic panels works almost the same way as using it on a metal body panel does. The main difference is that you will nowhere near the force on a plastic bumper cover as you will on a metal body panel. This is because the plastic of the bumper cover is much more pliable and resilient than a body panel is. If you are unsure of how much force to use, you should gradually increase the force until the dent is successfully removed. If you use too much force, you will need to block and dolly the reverse dent to remove it.
For more info, see How to Use a Car Dent Puller