Spotify → Apple Music

Open music.apple.com and sign in. Press ⌘ ⌥ I, open the Console tab, paste this, and copy what it prints:

(() => {
  if (!location.hostname.endsWith('music.apple.com')) return 'wrong tab';
  const c = document.cookie.split('; ').find(s => s.startsWith('media-user-token='));
  return c ? c.slice(17) : 'not signed in';
})()

It is sent with this one request to create the playlist, and is never stored or logged. It is not saved in this browser either, so you will paste it again next time.