Update 25 April 2020:
Many thanks to @KZL1992 @tomslominski @ptrofi @n-thumann for recent comments on their experiences with 20.04 and Parallels 13/14/15, especially the subsitution of the latest prl-tools-lin.iso
for older Parallels to get it going.
First off, credit goes to github.com/rudolfratusinski for leading the way here.
https://gist.github.com/rudolfratusinski/a4d9e3caff11a4d9d81d2e84abc9afbf