When creating a Smart Link, Pre-Save or an Action Page you can choose to promote either a single song or an entire Album/EP. If you wish to promote a specific song, you must grab the TRACK URL, and If you wish to promote an entire album, you must grab the ALBUM URL.
To grab the track URL on Apple Music:
As you click on the top field, you will be prompted to share the ALBUM:
This will result in: https://itunes.apple.com/us/album/joyful-noise/327536124
If you click on the individual track however, you will be able to grab the TRACK url:
(Notice how it now reads "song" instead of "album") :
This will result in: https://itunes.apple.com/us/album/dark-clouds-rain-soul-dub/327536124?i=327536133
Use this version of the URL when setting up Smart Link.
To grab the track URL on Spotify:
Click on the three dots next to the song, rather than the album, and then select "share song", and "copy link":
This will then produce this link for you to use, which is the correct version:
To grab the full album link, click the three dots next to the album (even if it is just one song), select "share album", and then selecting "copy link":
It will give you this link: https://open.spotify.com/album/0AfNRbX4a4X2llmfwHEG1H
You can use this link to promote your album!