One way to do this is to start a Magnet Mayhem race and when it starts wait for all the AI to place magnets on you and then when you can not see the majority of your vehicle then press the X button (whilst holding the A button) and then after 2 seconds let go of the X button (brake boost) and then all the magnets should come off. If you have more then 10 on you then it should pop up if you don't try again.
Remember after a while the magnets drop off by themselves. So try get them on as soon as possible.