Flarum has a feature called MediaEmbed that's not enabled by default.
You can either enable every supported website (includes YouTube and many others) using FoF Formatting https://discuss.flarum.org/d/17770-friendsofflarum-formatting
Or you can use the Selective MediaEmbed extension to enable it just for Spotify https://discuss.flarum.org/d/31404-selective-mediaembed
Some of the supported embed websites require some specific cross-origin frame settings to work fully. I'm not sure about Spotify, I think it should with just one of the 2 extensions mentioned and no other changes.
