How to get a Screen Normal direction?

I want to know how to get a Screen normal direction?

Please answer to me.
Thank you.

If you multiply 'Vector( (0,0,-1) )" by your camera’s position+translation matrix (aka World matrix) then that should be a vector that is pointing from the screen into your world in World space.

Are you doing this in the BGE or Blender proper?

Yes. I am a modeler an blender python programmer.
And thank you with your reply.