Do i put this script in the block or in the click detector?

Here is the script:

local re = game.ReplicatedStorage.ClickedButton 

function onClick()
    re:FireServer()
end

script.Parent.MouseButton1Down:Connect(onClick)

A ClickDetector uses the MouseClick not MouseButton1Down.

You can also use the event in a server Script inside of a part. You will then need to define your ClickDetector in some variable to listen for the mouse click.

Since this is running on the server you don't need to use a remote event at all.

(Server) Script:

local part = script.Parent --The parent of this script
local cd = part.ClickDetector --The detector inside of the part

cd.MouseClick:Connect(function(player)
	--run your code in here

	--This is running on the server side
end)

edit: note that this event includes the player instance that clicked as a paramater