Reinstallation of Installation of Endnote - product key lost.

I changed the computer from PC  (Win 10 ) to Mac (Sierra). My Endnote wersion was  Endnote 7,5.  

I would like to instal the program on PC  unfortunatelly I cannot find my product key 

I have still installed  Endnote in my PC -  Is there any possiblity to check it in my already working  endnote App  on PC ?

How can I get it?

All the best Paweł