Do you mean, the product key?
It's printed on the CD/DVD case label. In some cases, it's on a label glued on the case.
If it's a downloaded installer, you sould get it from the site from where you downloaded it, if it was not sent to your e-mail address that you provided before you started the download. THIS IS THE REASON you must enter a valid e-mail address.
If you did NOT get it from the official channel, then you are on your own...
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