To embed a Spotify song into a post, follow these steps:
Fire up the Spotify application and find the song you want to post. Copy the Spotify URI by right-clicking on the song title and selecting Copy Spotify URI. It may look something like this "Spotify :track:2Yk8tgWVPzpEm0seo3xle6"
In your post, position the cursor where the clip is to be placed and paste the URI directly, then Save (you won't see the image until after saving)
The result should look something like this:
If you don't have a Spotify account and don't want to create one, please ask a mod to post it for you.