You need to have a valid purchase code to be able to get an API key for the theme registration.

To obtain the purchase code:

1Go to ThemeForest and login into your account.
2In the upper right corner, hover over your username and click on Downloads
3 Scroll down to Jupiter, click on Download button, and then click on the “License certificate & purchase code (text)” link.
4In the text file, take note of the Item Purchase Code. It is similar to rt76ed7sd-0000-1234-9655-111111111111