I purchased the free rider on window, how do I use the VR mode.
The VR mode is mostly aimed at being used with Google Cardboard style VR viewers on Android. However it is possible to use it on a computer too.
You activate side-by-side VR mode in the Custom Settings.
You could for example use it with your FPV goggles connected via HDMI. You will probably need to adjust the resolution of the simulation to fit the goggles.
Note that it is not a complete plug-and play VR solution, it is mostly for those who like to tinker and experiment.
(You can find more general tips in the user manual, get back to me with specifics on how you intend to use it and I might be able to give more specific information).
If you're mainly interested in using your VR headset as FPV goggles, you can use whatever software your headset provides for mirroring your PC screen.
Or, a third party option like Desktop+
https://store.steampowered.com/app/1494460/Desktop/
You might also want to look into experimenting with software such as UUVR.