Every purchase on Envato Market comes with a purchase code. The installer asks for it, together with the site's name and the administrator's account.
Open your downloads
Sign in to Envato Market and open Downloads from your account menu.
Download the licence
Next to the product, choose Download, then License certificate & purchase code. A PDF or text version is fine.
Copy the code
The purchase code is a long string of letters and digits in five groups, like
1a2b3c4d-1a2b-1a2b-1a2b-1a2b3c4d5e6f.
Envato's own help page shows the same steps with pictures: Where is my purchase code?
Warning
Keep the code private. Don't post it in forums or paste it into support messages: we can see your purchase from your Envato account, so we never need the code itself.