Why Does Botox Make Forehead Look Shiny?

Smooth surfaces are more likely to reflect light uniformly and appear shiny. There is no other plausible reason for that.  Its certainly not related to the Botox itself.
Regards

Any of the oil or sweat glands can be activated by the neurotransmitter that Botox blocks. In some cases this can produce cessation of activity such as dexcreased sweating. But in other circumstances denervation can lead to increased activity of the oil glands. We commonly see this following surgery of the nose, for example following rhinoplasty. Surgery may disconnect nerves which normally travel to the oil glands of the skin. People commonly report an oily nose after nose jobs for this reason. Botox may act in a similar way by "chemically" blocking the nerves and leading to increased activity of the oil glands.