Using WebSockets in AngularJS / C# application on older Windows

We want to implement WebSockets in a web application that is being hosted on both newer and older Windows machines. We know that .Net WebSockets require at least Windows 8 and Windows Server 2012, but upgrading the old machines (Windows Server 2008, Windows 7) of multiple customers is not an option for us.

Our existing web application is a client-server framework, the front end is AngularJS communicating via HTTP request/responses with C# web api. We are thinking of using Angular WebSocket module for the front end, however, we are not sure what path to take for the C# api. Any recommendation would be appreciated.