The midi files are rarely given. I only know 3 artists out of dozens that give out midi files for their songs, 2 of which also gives vsqx/ustx files.

You have 2 options: find another cover that gives their files, or transcribe it yourself.

First option is still quite difficult, but there are more chances to get the midi, or even the vsqx files.

Second option requires some transcription skills. Get the song key and bpm and start transcribing. You can transcribe the sound directly in your vocalsynth editor or use a daw to transcribe with a midi editor and have a sound. In both cases, don't forget to import the sound itself using youtube to mp3 or any similar tool.

I prefer the second option, since in some vocal synth editors, you don't have full daw features (like utau/openutau). I like to use equalizers to get a better gist of the lyrical parts, but there are also AI seperation tools that you can use at your own discretion (certain harmonies can be missing, since these tools aren't perfect). Having an instrument (piano or guitar is recommended) is also pretty usefull

It seems complicated to do at first, but you'll start finding patterns and it will get somewhat easier. Don't hesitate to ask for help for transcribing if you need some.