To have i-screen integration enabled on your account, you will need to access Business Setup -> Integrations -> Click to Connect under the i-screen option to request access.
Please note: to use this integration you will need an i-screen account from https://www.i-screen.com.au/
You can access your api key from your i-screen account. Choose access tokens from the top menu bar. If you don't already have a token for SimpleClinic then click Add.