Question Capture object and its shadow in alpha channel

I am making objects against a simple background and saving the camera scene to a png file.
I can capture the objects alpha in the file, but I would like to also capture the objects shadow as well. (The reason being I am composing new backgrounds with the images in post processing and being able to carry over the shadow really helps the results)
Anybody know the setup for doing something like that?
Thanks!

 

If I understand you correctly and this is still relevant
You need to go to the material settings of the object on which the shadow falls. Then Surface Options -> Blending Mode -> set Multiply.