The locking clip is a completely separate piece that comes with the car seat, and holds both the ends of the harness and lap portions of the seat belt together. (howtoadult.com)
For a shoulder / lap belt, simply tighten the lap portion and close the lock off. (10bestranked.com)
There are those that hook like protrusions on top of the carrier about where the child's thighs hold the lap portion of the seat belt. (babieslounge.com)