The serial number for RealFlight 6 (and its updates like 6.5) follows a specific alphanumeric pattern:
: If you have lost your physical serial number but previously registered the software, contact RealFlight Support real flight 65 serial number updated
After installation, do not launch the game yet. You need to manually download the final update for RF6.5 (version 6.50.026). While the official auto-updater is broken, a community mirror or a request to Horizon Hobby support can yield the .exe patch file. The serial number for RealFlight 6 (and its updates like 6