To create vocals for a demo that use words and melody:
download and install OpenUtau. For my tests I was using 0.1.565
download a Diffsinger Voicebank, For my tests I was trying Tiger and Peiton.
In Musescore, File→Export select just the vocals and export as MusicXML format (uncompressed)
In OpenUtau (from the main screen):
Tools→Install Singer and point it to the zip. It will ask “does this look right”, hit okay.
Drag any dlls in and let them be installed.
Create a new project, File→Import Tracks, point to your export. You will get a part added - if you double click on it you'll get the full details.
You will now be able to select singer→Tiger and set the phonemizer to DIFFS EN (Diffsinger English)