This isn't too hard to grab! Simply get up to high speed (50kmph+ / 30mph+), then do a stoppie (
). When you are going at high speed and overbalance a stoppie, you'll slide along the ground for a few meters, so you can slide over the finish line.
You can also just balance a stoppie and over-lean, once you're close to the line, but I found that this meant I didn't make the finish line, or I would bail to late. The other guide (ramming into the finish poles) also work, however, I kept finishing and not getting the achievement.