Currently the Ubuntu/Debian mkpasswd command does not support yescrypt.
In order to use it anyway, we can use the ulikoehler/mkpasswd docker image to run the proper version of mkpasswd:
docker run --rm -it ulikoehler/mkpasswd
This will prompt you for a password and then echo the yescrypt encrypted and salted password:
$ docker run --rm -it ulikoehler/mkpasswd Password: $y$j9T$YzrfO5lQkDWahpz5pwYzg/$HzQoMYt.7E1jj.sd6OyYCGI/Qk6oGehNgz5uvY1qp59