I’m using FS2020 Store version and it doesn’t appear in the list of SteamVR games. However while in game you can open SteamVR settings (either in HMD or from the Win32 app window menu) and change it. Once you do it is permanently saved in the steamvr config file:
C:\Program Files (x86)\Steam\config\steamvr.vrsettings
Here is what my entry looks like:
"system.generated.flightsimulator.exe" : {
"disableAsync" : false,
"motionSmoothingOverride" : 2,
"resolutionScale" : 140
},