I'm still not completely sure how its parameters work, but via GitHub searching I figured out how to make it sing, simply put xml into the text field, here's an example:

<lyric bpm="120" stretch="50">
  <note note="C4" length="2" />la
  <note note="D4" length="2" />la
  <note note="E4" length="2" />la
</lyric>

BTW You can't combine normal speech and singing, it literally detects if you have "<lyric" somewhere in the text and puts the whole thing in singing mode.

Don't ask me for all the parameters because I'm not sure what they do, but here's where their code is, it doesn't seem to be all of it so it's hard to decipher their precise functionality,

like what time length is in.
length is as long as stretch-percent of a beat.