You will need to install a user script manager extension to install this script.
Clicking the button opens the game's page on IsThereAnyDeal
The right page is found in the background using ITAD's search + some additional automagics. This is nearly always accurate. In my testing I have only found one game where it opened the wrong page (HITMAN 3) and that was only because there is no ITAD page for HITMAN 3 as of writing this.
If you wish however, you can change an option in the Tampermonkey menu so that the button opens the search results page instead.
Currently, the ITAD button is only added to the top of the page below the "View Offers" button. So no separate buttons for different editions or addons/DLC/what have you.