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script.Parent = Instance.new("ClickDetector")
script.Parent.ClickDetector.MouseClick:connect(function()
    game.Players:WaitForChild("PlayerGui").ScreenGui.TextBox.Visible = true
end)

You can use ClickDetector and put it in the part so that you can make a function, go to this for more information : http://wiki.roblox.com/index.php?title=Making_an_onClicked_script

You could just put a SurfaceGui into the part, put a TextButton in the part, change the textures and all that, then put a script in the TextButton with this code:

function onClick()
    game.StarterGui.ScreenGui.Enabled = true -- Change the ScreenGui to whatever the Gui is called
end

script.Parent.MouseButton1Click:connect(onClick)

Make everything within the screengui is visible, then just untick the enabled box in the screengui properties

Make sure to Accept my answer if it worked for you. Please comment back if it is not working and I'll help you out.

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