For anyone else who wants to include the FBlueprintSessionResult in your code – if you are having issues where your code will not compile with errors about unresolved external symbols.
The solution to that issue is to add the OnlineSubsystem module to your [Project].build.cs file like so:
PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine", "InputCore", "OnlineSubsystem" });