Click on Library, right click name of game, left click CD key, copy to clip board, enter in the box.
Alternatively, when it asks you to enter a key, it will come up with a long list of letters. Delete everything including and to the right of the percent symbol.
https://steamcommunity.com/app/47790/discussions/0/3493132181459714500/
Teď vyzkoušeno, funguje.