Licence manager page ASP.NET Web Forms

We have an fairly complex enterprise ASP.NET web forms 2.0 app.

We need to deploy this application to our clients who are in closed VPN environment.

Application will be deployed to the instances of our clients(basically one instance is equal to 200 or more client PC's).

We need to create licence manager page where we can configure duration of one licence for all clients within one instance. If the licence expires there will be popup warning for clients that they cannot use the app anymore.

What is the best way to achieve this?