The following files have been adapted from ViewTube userscript 'protocol' [[1]] instructions. But I find no reason using such a complex script just for opening a video to mpv, so instead I opted out to using a bookmarklet. The files may be installed as follows:
user$ mkdir -p $HOME/.local/{bin,share/applications}
user$ cp notube.desktop $HOME/.local/share/applications/notube.desktop
user$ install -D -m 755 notube.sh $HOME/.local/bin/notube.sh
user$ xdg-mime default notube.desktop x-scheme-handler/notube