I am wondering how to add an Arduino JS extension to Scratch 2.0 online.

The arduino extension can only be used on ScratchX rather than Scratch 2.0, because it is considered “experimental”. Loading extensions the arduino extension is easy. When you load an extension, a link to the arduino one is provided with a convenient button in the “featured extensions” panel.
ScratchX will not store your projects online, so every time you want to edit your project, you will have to load it from local storage. Also, be sure to save, because I don't think it saves automatically.

Znapi wrote:

The arduino extension can only be used on ScratchX rather than Scratch 2.0, because it is considered “experimental”. Loading extensions the arduino extension is easy. When you load an extension, a link to the arduino one is provided with a convenient button in the “featured extensions” panel.
ScratchX will not store your projects online, so every time you want to edit your project, you will have to load it from local storage. Also, be sure to save, because I don't think it saves automatically.

yeah, because its hard to save automatically offline

You can shift-click the File button on online editor, then click load experimental extension. Or open browser console and paste the js into there