Use the "Forgot Key" or "Order History" section on the site where you purchased it.
FSRealistic is updated frequently to stay compatible with MSFS Sim Updates. Pirated versions cannot access the auto-updater, meaning the software will eventually break after a game update. How to Get a Legitimate Key fsrealistic product key
Keep an eye on SimMarket or FlightSim.to; they often run seasonal sales where you can grab the plugin at a significant discount. Use the "Forgot Key" or "Order History" section