I want to make my button go to the next song on like spotify. I have this key on my desktop keynoard but I want to use arduino to add it to my laptop
On a PC for spotify you need to send
Ctrl + Right Arrow or
Ctrl + Cmd + Right Arrow on a Mac
To use the Keyboard library you will need to use an Arduino with a USB-capable processor like the Leonardo or Micro.
The USB Human Interface Device (HID) specification is here:
and how do you do it when it is not focused on spotify
If you modify the Keyboard library to use the "Consumer" code page (0x0C) instead of the "Keyboard/Keypad" code page (0x07) you can generate the "Scan Next Track" (0xB5) button and "Play/Skip" (0xCE) button, one of which is probably what your keyboard has.
Here is someone who did exactly that back in the Arduino 1.0.5 days: