Replace your lost sling hook or add one to your coat.
The sling hook harness on the shoulder is supposed to be broken as shown int he picture. We cut off the claps part because it's not used.
Basically, when using a sling, it tends to slide down the arm. The sling harness is to assist in keeping the sling in place. It's made with a easy release cam buckle because the sling harness is not legal for all types of competitions. Therefore, you can easily remove the harness strap when not in use.