Steam key - find name of game if you already have it in library?

https://ibb.co/JCsnmhN

Alternatively, use the SteamDB browser add-on on the same page, it will do the work for you, read through the inspector and display the information without any additional steps.

Seems you already have the answer.
Anyway, an easier method, but it only works if you don't have the game installed, is to try activating the key form the steam client. The client automatically pops up the installation wizard with the game name if the game is not already installed.

Thanks everyone. For text referance if picture goes offline. In FireFox on Mac (should be similar on Windows or Linux) :

• load "store.steampowered.com/account/registerkey" web page
• on page enter Product Code, select "agree to the terms" but do not click Continue button yet
• right click on page and select "Inspect Element"
• go to "Network" tab
• on page click Continue
• on "Network" tab in list select line with POST ajaxregisterkey/
• select "Response" tab
• scroll to line_items, 0, and then to line_item_description
• there is the name of the game

I am basically use the steamdb addon for browser, but you can activate key in steam client, where you can see that this game is "to be installed after activation"