The fibaro app uses a plugin which builds the app.json. So the file you will want to change will be located in drivers/FGMS-001/driver.compose.json → when you run it will move the code into the app.json for you.
In order to ‘run the app’ you will need to install it from code. There is a thread on it here :
This is only needed as you are changing to code directly yourself, most people will never need to load an app with the CLI 
I did warn you about getting your hands dirty.
Good Luck! If you have any issues best to create a new thread so that this is just for Fibaro issues.
