Why there is no paste-a-YouTube-link box
A web page that fetches arbitrary YouTube URLs has to deal with platform terms, changing delivery mechanisms, access controls, copyrighted material, and server-side downloading. None of that is necessary for the musical transcription step. Keeping the page focused on a local audio file makes the product simpler and keeps the privacy promise clear: your selected recording is processed in the browser rather than sent to our server as a remote download job.
If the video is yours, export the audio from the original editing project or use a download method provided by the platform for your own content. If you have permission from the rights holder, work from the authorized file they provide. After that, MP3, WAV, M4A, AAC, OGG, FLAC, and WebM audio can be used as input. The converter is a transcription tool, not a mechanism for obtaining media you are not entitled to copy.