Do not really understand but a few things:
You can get the OAuth Callback URL displayed by pressing on “OAuth Callback URL” underneath the “Connect” Button
Does everybody in your company have a different n8n instance? Because in this case it would be best to set the OAuth App credentials on a global level via the overwrites here:
Like that everybody in the company could use the same OAuth2 App from LinkedIn but they would not see the Client ID and Secret and all they would have to do is to press “connect” and then select their account like in any other application. This will however only make sense once the LinkedIn node got released, as an overwrite of generic base OAuth 2 credentials would not be a good idea.
The good news there is that the node will probably be released in the next days.