How To Add Weights to Necklaces?

Hi. On the body only breast bones are responsible for morphing, other changes in shape are handled through uv_1 and vertex paint.
To make a necklace morph like an area of the body it covers, it must receive uv_1 from this area. However, it does not make sense for a solid object to change its shape quite like the body does, so some of these cc necklaces may intentionally have this feature disabled.

Anyway, it you want, you can try transferring uv_1 from the body
- if using 2.7x blender, you need to expand the rig and make top selectable. If using blender 4+, you don’t need to do this;
- select the necklace, add Data transfer modifier;
- choose top as Source object;
- enable Face corner data, then UVs;
- change Mapping to Nearest face interpolated;
- expand UVs, choose uv_1;
- just in case the item has no uv_1, click on Generate data layers;
- apply