I read that but I don't know what a steam key is (most google searches I tried produced endless posts about getting dodgy one's etc) - I have an entry on the DLC tab of steam properties option for the game and that has a line about forum code DLC, but there is no apparent way to get at the value.
[Edit] Looking at this again, I also bought Cities in Motion and this also has a DLC entry for forum registration key. The difference is that r-click on game in steam library for CiM gives option to view CD Key. For KA2 it does not.