To get RTMP protocol data in ASP.NET Core, you can use the Microsoft Azure Media Services API with the .NET SDK. Live events are responsible for processing live streaming content. A live event provides an input endpoint (ingest URL) that you then provide to a live encoder. The live event receives input streams from the live encoder using the RTMP/S or Smooth Streaming protocols and makes them available for streaming through one or more streaming endpoints. Live events also provide a preview endpoint (preview URL) that you use to preview and validate your stream before further processing and delivery. More detail info, see https://learn.microsoft.com/en-us/azure/media-services/latest/stream-live-tutorial-with-api .
And you can also try to use rtmpsharp, which is a fast and lightweight data-oriented RTMP(S) library for .NET Desktop and .NET Core.