How do you fade the volume at the end of the song?
Welcome to Hookpad!
I’m afraid that Hookpad is a composing tool, not a mixing tool. Its built-in mixer only has the track and project volumes as well as a ‘Click’ and ‘Compressor’ setting.
However:
Exporting your song to a DAW that does have this feature is a viable option! 
Yes. I think the easiest way to achieve this is to do a wave export of your song and create the fadeout in a DAW. Another way would be to create a band change marker in Hookpad for each bar of your desired fadeout and then turn down the volume sliders for each instrument a bit more in every bar. But that’s a lot of work and not a great workflow.
Thanks for the information. I understand this is a writers program and I really enjoy working in the program. I am still learning as I go. As you folks are way more “computer smart” than I, I find it unusual that the program has no “easy” option to write in Dominate 7ths without having to borrow chords etc. That seems a bit cumbersome. Also there are no country, blues or Old Rock and Roll rhythm options.
Just food for thought on perhaps the next upgrade.
Thanks for all you do,
Michael