Steps for iPhone: Youtube to mp3 - https://x2convert.com/en140/download-youtube-to-mp3-music

^After the conversion is complete, click VIEW, then copy the provided link.

mp3 to base64 - https://base64.guru/converter/encode/video

^Change the datatype from Local File to Remote URL, then click “Encode MP3 to Base64.” Copy the base64 text provided from the website and paste it into the text box in the shortcut. Be sure to make this shortcut available on your watch! Inspiration from u/BennyM42