Background Object Hider/Shadow
I'm going nuts trying to remember the product. It has various shapes (like primatives) you can add to a scene. You can set it so the object you add is invisible, but will allow a shadow to be cast against it. (Example, if you're using a jpeg as a background, and it has a wall, this would allow the 3d objects in the scene to be cast on to the wall.)
I thought it was here on DAZ, but having trouble finding it because I have no idea what to call it.

Comments
Ah, I just found it.
https://www.daz3d.com/iray-hdri-toolkit
It's a great product... if you can remember what it is called.
Thanks for the heads up on this... that is a pretty cool asset!