Question transparent object's shadow strength

Transparent object's can cast shadow if the shader has a enabled ShadowCaster pass. But the shadow strength is the same to both opaque and transparent.

For example, the left cube is opaque and the right is transparent, but their shadow is the same.
I'm trying to make the transparent cube's shadow lighter, to make it strength smaller.



So far I've tried:
1. Light component's shadow strength, but it will changes opaque's shadow strength too.
2. Modify the alpha value in ShadowCaster, but it seems ShadowCaster only cares if the alpha value exists. If alpha >= 0, the object has shadow. If alpha < 0, the object has no shadow. Alpha's value doesn't change the strength like the Built-in pipeline.

Is there a proper way to change shadow's strength for transparent objects?