SMC can't use YT/TCRF links because of the way the sites are coded (or something similar to that.) If you want a custom song, you have to use Dropbox or File Garden. However, the URL format for those is different.
EXAMPLE: https://dl.dropboxusercontent.com/scl/fi/te2dz9er7kvaioyaie0eh/UFO-50-Maria-s-Theme-from-R...
All that's needed is for you to upload the audio file to dropbox, set privacy settings to "ANYONE WITH THE LINK" (or similar), set "www" to "dl", and add "usercontent" after the word "dropbox".