Simple plugin script for detecting clicks not working?

I'm trying to create a custon rotating tool plugin, and my first step is detecting clicks.

However, this script which is supposed to print text for every click, doesn't work, not even returning an exception.

local plugin = PluginManager():CreatePlugin() 
local toolbar = plugin:CreateToolbar("Test")
local newScriptButton = toolbar:CreateButton("Test", "Test", "rbxassetid://1507949215")

local function onNewScriptButtonClicked()
    local mouse = plugin:GetMouse()    -- Gets the PluginMouse 
    mouse.Button1Down:Connect(function()   
        print("Button 1 pressed from PluginMouse")
    end)
end

newScriptButton.Click:Connect(onNewScriptButtonClicked)