As there doesn't appear to be one by default when creating a new one. You may need to create the custom music folder within the Sims 4 folder. I created a new Sims 4 folder to test, as I didn't see a Custom Music folder in my current folder. Playing in Windowed Fullscreen or Windowed mode will allow you to minimize the screen easier when you need to.
To change the Display type, in game go to to Options > Game Options > Graphics look for Display type and in the drop down window, you can choose between Fullscreen Windowed Fullscreen and Windowed.
