Step 1. Visit the LinkedIn Developers site.
Step 2. Choose the option 'Create Application.'
Step 3. Enter data in the mandatory form that is going to pop up next to the button 'Register new application.' Don't forget to keep the URL format like that:
https://.supplysample.com. Mention the real company's name, describe it, upload its logotype, and leave contact data.
Accept terms and conditions after you're done.
Step 4. Click 'Submit.' This step is going to secure your API Key and Secret Key.
P.S.: Having a Redirect URL isn't compulsory when it comes to LinkedIn.
This is a short instruction for getting your authentication keys.