We support Youtube videos only for Self Transcription. When you create a new self transcription, you should be able to select Youtube as the option, paste a Youtube URL to load the video, slow it down and transcribe it.
For automatic transcription, we require the files to be on your computer so they can be uploaded to our servers. There are online youtube downloaders that will let you download an mp3 file of the the video to your computer. That said, please ensure that you're not violating copyright law in your jurisdiction when you use these tools.