Following this page to get a path for the Content directory, I was able to load a text file from Content/Text/Preamble.txt while in the editor.
FString Path = FPaths::ConvertRelativePathToFull(FPaths::GameContentDir());
Path += TEXT("/Text/Preamble.txt");
FFileHelper::LoadFileToString(Preamble, *Path);
However, when I package the game, the file isn’t found. In Project Settings, I’m including the directory as an Additional Non-Asset Directory to Package, and I also tried Additional Non-Asset Directory to Copy, but neither worked. Is there more that I have to do for it to be packaged?