You consider the thing the wrong way.
Dropshadow is not the starting point but the end of your journey.
First you have to talk with a object that may have a drop shadow then either get the value or set a value.
for example, assuming that you select a object that has a drop shadow,
you can use
app.selection.transparencySettings.dropShadowSettings;angle=120/* a value between -180 & 180)*/;
I try to apply a drop shadow on a clear element and it's true I failed. in fact, datas seem to be considered but not applied in real time to the selection. But if I apply a drop shadow manually, the object get the value I ordered formerly.