The first step is to go to: https://www.linkedin.com/developers/login and log in there with your LinkedIn account.

You'll be logged in then in the LinkedIn developer control panel and to add a new app, you have to click on the blue Create app button at the top right corner.

You'll see then the form for creating a new app and you have to enter there the app name (you may choose any name you prefer like Log in app for your website etc.), enter your company name, app description, logo and privacy policy and click on the Create app button at the bottom of the form.

This will create the app and LinkedIn will redirect you to the app management page. The final step is to click on the Auth tab at the top -

, scroll to the bottom of the form that will open and in the OAuth 2.0 Settings enter in the Redirect URL field the following url:

https://www.yourdomain.com/index.php?mod=login-linkedin

(by replacing www.yourdomain.com with your domain name / the one on which the script is installed) and click on Save.

You can then copy the App Client ID and Secret and copy them in the script administration panel and the Settings->Configuration Options page, which will enabled the LinkedIn log in for the users.