This will return the directory where the game executable is stored. However this may not always be useful, particularly as some devices run the exe from a *.zip file, so this would return the same no matter where the game is actually running from.WARNING! This function may not work as you expect due to GameMaker Studio 2 being sandboxed! Please see the section on File System Limits for more information.
dir = program_directory;
This will store the directory where the executable is stored in a variable.