How to Find Your TrackMania United Player Key on the Steam Version

Look for the InstallKey=****-****-****-****-*** value in the Nadeo.ini file located in your Steam install folder for TrackMania United.

The stars will be alpha-numeric characters (letters are capitalized/upper-case)

The default installation paths are:
32-bit Windows
C:\Program Files\Steam\steamapps\common\trackmania united\

64-bit Windows
C:\Program Files (x86)\Steam\steamapps\common\trackmania united\