If you're using the iOS app and are wanting to add a practitioner's API key there, you can follow along with this article here: http://help.finger-ink.com/en/articles/1737916-adding-additional-api-keys.
If you're using web forms and are wanting to add a practitioner's API key to the portal, here's how:
Each practitioner should generate an API key from within their own Cliniko account.
Sign-in to the Finger-Ink portal using the API key: https://portal.finger-ink.com/cliniko/sign-in
When prompted, create a password for future sign-ins.