CC Necklace Help

The mesh must move and morph the same way the area of the body it covers does that.
You need to transfer weights and uv_1 from the upper body. Right now it is just assihned to one bone, does not even morph
That said, it will not behave right regardless. Each of these elements the chain is made of will distort individually because that is what morphing and moving is basically ( mesh changing its shape). The more vertices there is, the less smooth it happens. Chains like this should be made through textures, then distortion will not happen.
- expand the rig, make top selectable;
- select the chain, create a new map, name the first uv_0, the second to uv_1;
- add data transfer, choose top as source object, set the rest like in the pic, apply;
- select top, shift-select the chain, transfer weights with these settings, then use clean with all groups and limit total;
- import the mesh in s4s, check in-game, change the width of shoulders/neck or the size of the chest to see what i mean about these things distorting