I had to add another location for my simbrief downloader to copy the flight plan. Below are the two I use. Your exact folder will be different based on you userID and MSFS install folder, but should be fairly close.
500/700: C:\Users\userID\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalState\packages\aerosoft-crj\work
900/1000:
C:\Users\userID\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalState\packages\aerosoft-crj-900-1000\work