Force shadow directional light

Hello,
I have a script on my Camera which writes a custom depth in the depth buffer. For example to make appear a plane. I would like to get the shadows of the other gameObjects in the scene on the “plane”, these objects are not visible by the camera. But because they are not visible, and there is nothing in the frustrum of the camera, the shadows are not computed. To force shadows, I must create a small object invisble, is there another way ?

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