Lost product key for my Endnote 8

I changed the computer from PC  to Mac (Sierra). My Endnote wersion is 8
I would like to instal the program on my new MAC  unfortunatelly I cannot find my Lost … 
However I have still Endnote installed in my old PC  Is there any possiblity to get a product key  from my  already working endnote App on PC?
How can I get it?

Thank you