Connect Windows Software to License Keygen Web Application (API Integration)

I have a license key generator web application and a Windows-based software. I want to connect both, so the software can validate license keys during installation or startup. The connection should be secure and support basic licensing logic (e.g., verify, activate, expire, etc.).

Requirements:

Build or improve API in my web app (PHP or relevant backend)

Modify Windows software (C# / VB.NET / other) to connect with API

Implement license verification and validation

Handle error messages if license is invalid/expired

Optionally store local activation data for offline check

Deliverables:

Fully working integration

Documentation of API endpoints and how it works

Support for minor fixes if needed