In DAZ Studio? You need a shadow catcher shader on the ground plane - pwCatch will do the job, or you can fairly easily make one with Shader Mixer in DS4 Pro.
I haven’t checked this works, but you could also, if you are going to be editing the image, try setting a ground plane’s ambient and diffuse colours to 100% strength, white colour in the Surfaces pane with 0 specular strength - render once with the plane, and once without - save as png. Now place the version with the plane over the background you want to add it to and set its blending mode to multiply. Finally place the without-plane image on top of that, with normal blending mode.