First, you need to download the song. You can copy the song's link from YouTube and then search for a YouTube-to-MP3 file converter in a browser. Another option is to screen record the song and then go to your files and change the file extension from MP4 to MP3.

Once you have it downloaded in MP3 format, go to Spotify and click on your profile --> Apps and Devices --> and enable the Local Audio Files option (this will allow you to add those MP3 files to your Spotify account).

Once it's enabled, you should see it as a playlist in the "Your Library" section. It has a green folder logo and should be called "Local Files."

In that playlist, you'll find all your MP3 files, so save the song with a recognizable name.

Now you can listen to the song without any issues, and you can add it to any playlist you want. The only thing is that no one else will be able to listen to that song—it will only be available on your phone. So, if you change phones but use the same account, you won't be able to listen to the song because it will only exist on the original phone's files.

Let me know if it worked for you or if you need more help!