If you still have the old device with Fusion 8 installed, you can go to "VMware Fusion" menu ... "License...".
Or you can log in to your VMware account (which on another thread you seem to be having trouble with) to retrieve the licence key.
If you've got a new MBP and want to install Fusion 8 then I'm afraid that that most likely won't work.
Most new MBP will comes with High Sierra and for macOS HS you need Fusion 10.
There's a possibility to use Fusion 10 as a 30 day time limited trial to see if it works out for you.
If it works out for you then buy and assign the license to the install and the time limit will be removed (so no need to reinstall)