How do you put an ImageButton into the Topbar, or the CoreGui?

So normally there'd be no way to check for click events when a button is behind the topbar and roblox has no intention of adding this as a feature. But there is a hacky way to get around this.

What we do is whenever the player has clicked somewhere on the screen, we check the click's position to see if it is inside the button behind the topbar!

--variables
local uis = game:GetService("UserInputService")
local mouse = game.Players.LocalPlayer:GetMouse()

--button (change this to your button)
local button=script.Parent.ScreenGui.TextButton

uis.InputEnded:Connect(function(input)
    --checks to see if the the mouse is inside the button's boundaries
    --if so then
    if input.UserInputType == Enum.UserInputType.MouseButton1 then
        if mouse.X>button.AbsolutePosition.X and mouse.X<button.AbsolutePosition.X+button.AbsoluteSize.X
            and mouse.Y>button.AbsolutePosition.Y and mouse.Y<button.AbsolutePosition.Y+button.AbsoluteSize.Y then
            print("clicked button")
        end
    end
end)

thus we get this

Wiki on UserInputService Wiki on AbsolutePosition