Just would like to add a solution I stumbled upon to making the Media Player Keyboard function button open up spotify (even without your default media player being Spotify)
Heres how to do it:
1) Open regedit.
2) Navigate to the following:
3) Create a new string (REG_SZ) in folder 16 and rename it to ShellExecute
4) Right click on ShellExecute, click modify
5) Paste the following in Value Data: (note:replace the YOURUSERNAMEGOESHERE with whatever your user name folder is)
6) Close regedit and try your Media Player Function button (If it doesn't work you may have a different installation or exe location)
I hope this helps a few people! I had to pry around different forums on the internet to figure out what to do as it wasn't easily explained.