You can create a new custom shortcut
Go so Settings > Keyboard Shortcuts > Scroll all the way to the bottom
and click on the +

Assign a name like "Spotify Play/Pause"
In the command, paste this:
dbus-send --print-reply --dest=org.mpris.MediaPlayer2.spotify /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player.PlayPause

Change the last word "PlayPause" to:
Previous, Next or Stop deppending on the command you want to execute

Set the keys to execute the command and you should be good to go