Just subtract the location of the top left corner of the screen in the world from the location of the point in the world.
If your world is 1000x1000 and your screen is 100x100 and the top left corner of the screeen is at 200,200 in the world when the player clicks the center of it, then the point they clicked is at 50,50 in the screen and 250, 250 in the world, and if you then move the screen to the right by 50 pixels so it is as 250, 200, and then subtract that from the point's world location of 250, 250, then you are left with 0, 50, which is the point at which to plot the point on the current viewport.
If your world is 1000x1000 and your screen is 100x100 and the top left corner of the screeen is at 200,200 in the world when the player clicks the center of it, then the point they clicked is at 50,50 in the screen and 250, 250 in the world, and if you then move the screen to the right by 50 pixels so it is as 250, 200, and then subtract that from the point's world location of 250, 250, then you are left with 0, 50, which is the point at which to plot the point on the current viewport.