Adjustment layer -> Effect: RGB Curves, with a slight drop in exposure. Make a mask in the desired shadow shape, and feather for nice blurred edges. Keyframe the mask from right to left. Crack open a beer.
Am I misreading the post? Are you trying to make something more complicated?
On a flat surface the suggested solution would work without any issues, however, if I read correctly your image, you have some drapes in the shot, so the shadow would have to follow the volume of the drapes. One easy way of doing that would be to animate the shadow like suggested as it would be on a flat surface, precompose that and use Free Form V2 or FreeForm Pro on it together with a displacement map (which you will have to paint to match the volume of the drapes) to displace the shadow and have it follow the volume of the drapes.