How to use this add-on:
1. You make sure your cursor is in a text input box inside the window – not the address bar or search bar.
2. You press and hold the Ctrl. key.
3. While Ctrl. key is pressed, you hit the apostrophe key ['] – not the backtick (grave accent) key [`]
4. You punch the keys corresponding to the Unicode of your character.

Example. If you want to type "CO₂", you type:
1. [SHIFT+C] to input capital letter C
2. [SHIFT+O] to input capital letter O
3. [CTRL+'] to activate Unicode input
4. 2082 to input the Unicode of subscript two

Note: this add-on does not work in the "Write a review" window or in YouTube comments (but it works in YouTube search field). The author seems to not care about the shortcomings of this add-on.