You need to add a socket on the hand bone, you can do it from personal probably (right click on the bone). That socket is attached to the bone but you can modify relative transform as well. There is a useful option (dont remember but probably right click on the created socket) to attach a preview mesh, so you can check how it will looks. But this is only for preview, you will have to do the attachment manually for your character at start.
Better explained here: https://docs.unrealengine.com/en-us/Engine/Content/Types/SkeletalMeshes/Sockets