How do I obtain a Parallels Desktop 8 license?

Dear Administrator,
I downloaded a copy of Parallels Desktop 8 installer disk image and installed it. But, there is no option how to activate it. I don't have a license for it, and the software doesn't let me get a trial license. Please let me know how I can purchase a license for it. There is the subscription option for a license for $79.99/year. I am wondering whether I can just purchase a permanent single user license for a new version of the software and use that license code for Parallels Desktop 8? My Mac OS X 10.6.8 does not support Parallels Desktop 10,11,12, or 13 that is available for purchase on your website.
Thank you very much for your time and effort in this matter.
Sincerely yours,
Zigang Pan