When you have installed and enabled the plugin and you have setup the custom field, you can start using it.
Go to the article you like to add the Spotify
You see a new tab. In this example it's called "Joomill Custom Fields" but it's the name you choose for the the Custom Fields Group.
You also see the description if you entered one in the Custom Fields Group.
Spotify: Here you enter the Spotify URI
You can get the Spotify URI at the user, artist, track, album or playlist page on spotify.
You van find it at the ... - button --> share --> copy Spotify URI