They are called "J" hooks. Like chains, they should be dropped through the stake pocket and brought up on the outside and hung on the pocket. Typically, this will subject the webbing to sharp edges so the webbing should be protected. Old fire hose, commercially available leather sleeves or even strips of leather from those old nasty shoes you just tossed into the trash will work.
Anytime a hook of any type is simply catching the underside of a pocket, rub rail, frame, whatever, you would be wise to put a bungee on it to keep it hooked should your tie down slack.