Copy a track's link from Spotify — ⋯ → Share → Copy link to song — and paste it onto an empty line in the editor. It becomes the player straight away, and it plays there while you write.
You can also press /, pick Video or embed, and paste the link into the
block instead.
Links look like open.spotify.com/track/…. Track links are what the embed
takes; an album or playlist link will not turn into a player.
See Video and embeds for everything else that works this way.
Worth a thought
A music player is a large, slow thing to put in a travel post, and it plays only a preview for readers who are not signed in to Spotify. It earns its place when the music is genuinely part of the story — the song that was on in every bar that summer — and not much otherwise.