How do I make a camera's view a texture(screen)

Im trying to make a sniper scope so I wanted to make the scope a camera view by making it like a screen on the scope.

what I have                                              what I want

ok so i found out how to do this heres how i did it
i created a render texture and put that on the scope.
then I created a camera and set the render texture from that to the texture

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