I know how to do it in Resonite, but I've not dabbled in Unity much at all, so I can't say how dynabones are even done there let alone on VRC.
But, if it's anything like on Resonite; you first parent the start of the chain to one of the cuffs chain loops, then give the other cuffs chain loop a 'child' object which will be the anchor to the end chain bone of the chain armature. In Resonite (therefore likely in Unity as well), the dynobone config menu has a slot where you assign the anchor as well as how many bones that are in use, which I think in this case is 13.