Connecting script in workspace to script in StarterGui?

I have a script in a block in workspace that turns on w/ a clickdetector. Ideally I need it to change the transparency of a ScreenGui in StarterGui to 1 when clicked, but I don't know if there's any way to connect the two.

This is the script in the clickdetector, but I just don't know how to connect it with StarterGui. If anyone could help I'd really appreciate it. :)

local isOn = true
StarterGui = StarterGui.ScreenGui.Frame.ImageLabel.ImageTransparency

function on()
 isOn = true
 script.StarterGui = 0.8

end

function off()
 isOn = false
 script.StarterGui = 1

end

function onClicked()

 if isOn == true then off() else on() end

end

script.Parent.ClickDetector.MouseClick:connect(onClicked)

on()