I did it.
WINDOWS 10 (not sure for other OS)
I removed the Windows App store version and downloaded the Windows Version from Spotify
(I searched all spotify files *spotify*.* in file explorer and deleted them, but you may not have to do this, I also ran cCleaner)
sign in (very important to create the app link in appdata for later)
I got an MP3 file, Right clicked and open with. I then hovered over open with another program... Selected "Choose another app"
Click More Apps......
Scroll to the bottom... click... "Look for another app on this PC"
then go to the folder containing Spotify (to get this, Right click the spotify Icon and then properties) Example - C://Users/usersname/Appdata/roaming/spotify/spotify.exe Then Clicked open. Nothing happens (don't worry)
Now goto... PCSettings, Apps, default apps, Music Player (should be Groove or Windows) Click it, Select Spotify. Done. Now when I press the Music button on my Keyboard Spotify loads.
If you have a computer for multiple users, you may have to play around as it took ages to get to work on my partners. but on my PC worked first time.
Hope this helps those that just want to click 1 button and load Spotify from their Keyboard etc. feel free to share elsewhere.