Thank you all for your comments and clarification!
With all of this extra time at home, I will be going down the path of Python.

I found this in c4d.BaseObject in the documentation:

import c4d obj = c4d.BaseObject(c4d.Ocube) # Create new cube obj.SetRelPos(c4d.Vector(20)) # Set position of cube doc.InsertObject(obj) # Insert object in document c4d.EventAdd() # Send global event message

Now to convert 2D mouse coordinates to 3D and input those (on click) to the obj.SetRelPos(c4d.Vector(x))... I know this is an x,y,z vector, as this code generates the cube at 20,20,20. And it looks like the code works with centimetres, not inches, even if my units are in inches. I may need to do some conversion down the road...