It's right there but you can't see it. Pull the seat cushion to the side to reveal :
The hole with the strap in it. You can barely see the strap in there. Pull that out and yank in it.
The problem is the strap isn't anchored well so it can fall all the way in that hole. If it does you'll have a hell of a time getting it out. Especially if your hands are any larger than mine. I can barely get two fingers on there to fish it out and I have smaller hands.