A bit confused as to what you mean, but if you mean having a shadow under the white box, you can go to the white box layer property -> Drop Shadow and change the distance to 0, the size to around 4~6 (up to you, really), and change the opacity to make it more subtle or strong.
If you mean shadows "peeling" under the white box corners, I usually grab the pen tool and create a peeling shadow look with it, and then I apply gradience to it (darkest towards the white box, lightest towards the end). This should be placed under the white box layer
You can also duplicate the white layer, invert it (ctrl + i), and guassian blur it. Stick that under the white box layer, and you've got a nice shadow effect that you can alter easily.
I probably confused you, but your question was confusing itself