Just bought it. The description says go to Cyberpunk 2077 click on "more" see key and use it on another side.

But there is no "more" (or anything) to click when I go to my games on the gog page. in fact there is a sale offer for the game, despite it says its already in my library (since a while).

So what do you think do I need to do to add the DLC map? how to get to the serial key of the map to redeem it on the external site?

works now, after the special sale is gone from my page (already got cp anyway), I can click on "more".

The code gives you extra features for the free Piggyback interactive map. It's not in the game itself, but rather, on a web page. Here's the instructions for redeeming the code.