To install the Artlist Library Extension, you will need the Artlist Hub. You can download the Hub from the Tools page.
After this, installing is simple! Log in to the Hub and it will automatically install the Artlist Library for you. The Hub will check you have got Premiere Pro on your system and once finished, the installation process will be complete.
Now all you need to do is restart Premiere, head to Window > Extensions and select Artlist Library!
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