How can I get an invisible object to cast a shadow?

Currently I'm using a shader/material combo that is somewhat transparent and does not cast shadows, even though I have them enabled on the object.

I'm using a procedural mesh modified by perlin noise during-run time that I'd like to cast shadows and still use the shader I'm currently using.

I figured there's 2 ways to achieve this, modify the shader or create a 2nd object directly on top of my transparent one with an opaque shader/material combo to cast the shadows. This object would need to be invisible to the camera though.

How would I go about doing this?

the image above shows an example of the scene I'm working on. The black circular shape does not cast shadows. Is there a way to make an object invisible to the camera at all times yet still cast shadows?