Do that in your rendering software. That function is not within the scope of AssetForge.
Viewing post in Shadow on transparent ground
Well, rendering sprites is in scope of AssetForge. Shadows in renders are in scope of Asset Forge. I ask for plane that is transparent but catches shadow like any other object in AssetForge
Of course I can do it elsewhere, but it complicates workflow when I can render sprite with few clicks in AssetForge itself.