They suggested Radiesse for neck, but I don't see many examples?

no this is not the right approach, you likely need skin tightening, lasers, and MNRF and threads or a surgery. fillers like radiesse will not help that much maybe some texture but very little. I suggest you post photos and consult. Best, Dr. Emer.

Hyper-dilute Radiesse is Raddiese which has been diluted with saline. It has the effect of lifting and tightening by producing collagen, but Hyper-dilute Radiesse does not replace volume like Radiesse does.
I would use Hyper-dilute Radiesse, then wait six weeks and use PDO threads in the neck. I would reassess patient in six weeks and based on result, may offer more Hyper-dilute Radiesse if needed, waiting another six weeks and reassess and possible use more threads.

With weight loss you'll have excess skin. At a minimum a minimally invasive therapy such as FaceTite or Morpheus8 would be necessary to tighten the skin. A neck lift would be optimal.