If you are still on 3ds Max 2018, you cannot use OSL shaders natively. Instead, use the VRayDirt in "Ambient Occlusion" mode inverted, plugged into a light’s Gobo slot. It’s clunky, but it works.
For users who want a button-click solution, three scripts dominate the Max 2018–2023 market: dappled light generator for 3ds max 20182023 f better
Before buying software, try the Scatter + Opacity Map technique—vastly improved in Max 2020+. If you are still on 3ds Max 2018
Step-by-step:
Verdict: Free, but lacks depth. Shadows look flat because the opacity map is 2D. Verdict: Free, but lacks depth
Architects and visualizers currently face three critical hurdles when creating dappled light: