You will be presented with following screen. Go to Setup -> OpenID Connect on the navigation bar on top.
Click on Generate New Client API Credentials button.
You will be redirected to the app details page. Fill in the required details and enter Authorized Redirect URIs from miniOrange Oauth Client plugin which you copied in the last step and click on Generate Credentials.
You will be brought to App details page. Here, you can find Client ID and Client Secret
Copy these credentials and keep it handy because you will need it in the next step.
You have successfully completed your WHMCS App OAuth Server side configurations.