Spotify Volume Controller lets you hook two keys (for example your volume controls) directly to Spotify using Spotify's REST API.
To use Spotify Volume Controller you need to create an application using the Spotify Developer Dashboard.
After creating an application go into its settings and add a Redirect URI. http://127.0.0.1:5000/callback is default callback URI but you can use whatever you want as long as you set it in the config file and follow Spotify's requirements. For example localhost is not allowed to be used.
Then copy the client secret and the client id values to the config-example.json file as strings. After that rename the file to config.json and start the program. Follow the instructions and if everything is correct it should be able to authorize it self.
If the default values aren't working or you just want to use different keys just change the volume_up and volume_down values in the config file.
The values should be the decimal value of the virtual key codes for the keys you want to use. See Microsoft's documentation for a complete list of virtual key codes. You can also set print_keys to true to print the decimal values of the virtual key code for each key you press.
This seems to be some kind of rate limiting from Spotify (though they have a separate error code for that, 429, so not sure why this one is 403). If you are encountering it frequently, try to increase the volume increment using the volume_increment config option (default is 1).
See the BUILDING document.