rFactor2 must send the game data as shared memory. A plugin is required for this. If it is already installed, this step can be skipped. Otherwise go to
https://github.com/TheIronWolfModding/rF2SharedMemoryMapPlugin
download the .zip file (there is a download link in the middle of the page) and unzip it.
The unzipped archive contains a .dll file (rFactor2SharedMemoryMapPlugin64.dll). This file must be copied into the rFactor 2\\Bin64\\Plugins directory. For me the Steam directory is set to d:\\games, so the file must be in this path: D:\\Games\\steamapps\\common\\rFactor 2\\Bin64\\Plugins
Now start rFactor and click "Options"
Select and activate the plugin. The game must be restarted now
Now launch the client and race!