When it comes to software, it's important to distinguish between a license key and a license agreement. A license key is primarily used to prevent unauthorized copying of the software, which may not be a major concern for your web application. On the other hand, a license agreement is a legal document that outlines the terms and conditions users must adhere to when using your application. It seems like you're more concerned about the former.
For a web application, implementing a secure login system is crucial. You should only provide access to users who have paid for your services. If you plan on licensing your API to other users, consider assigning them a unique GUID (System.Guid.NewGuid();) and requiring this GUID as a parameter for all API calls. By verifying if the GUID is linked to a paid account, you can control access effectively. I recommend using an HTTPS connection for added security.
If you intend to sell your web application for users to host on their own servers, incorporating a license key would be beneficial. You could add the license key as a parameter in your <appSettings> section in the web.config file. There are various methods for generating license keys, typically involving creating random numbers until a specific check-sum requirement is met. While simple check-sums like 1253-38 (1+2=3, 5+3=8) are easy to crack, you can develop more intricate check-sum algorithms for enhanced protection.