You still need the product key. Downloading the installer alone does not give you the software for free; it simply allows you to install the software you bought on a disc onto a computer that has no disc drive.
as a standalone software purchase. This version was originally distributed via DVD, and unlike newer versions (like RealFlight Evolution), it was not packaged for digital-only marketplaces like Steam. RealFlight Forums 🛠️ How to Obtain or Reinstall Realflight 7.5 Download