Hi,
Please, first carefully read this introduction to OpenVidu and how it fits in your application: Developing your video app - OpenVidu Docs. Your .NET code would be Your server application in the description of a typical OpenVidu application architecture.
Then, you can check out the .NET tutorial: Application server - OpenVidu Docs
This basic tutorial is just an easy template to use as a backend for your OpenVidu application. You can adapt it to your needs.