The beautiful thing about NodeBB is that you do not need a plugin to achieve this specifically since it's already built in.
You have at least 2 options both in the Admin Control Panel (ACP):Extend > Widgets
There is a HTML widget
Here you have control over the footer, header and many other areas and can control globally or on a page by page basis.
Custom Content >
Here you can input the code, save and then watch the "live reload" do it's magic.
Fun fun fun! 😳