Hiya!
If you view your projects through the dashboard: https://itch.io/dashboard
And then click 'more' to the right of the game listing, you can find additional options to create keys that you can then distribute to people who can gain access to your paid game!
Edit: This link here also explains a bit more about keys in depth so you get a good idea of how they work and how versatile they are! https://itch.io/docs/creators/download-keys