Explanation
Box 1: Stream
The File.OpenRead(String) method opens an existing file for reading.
Example: Open the stream and read it back.
using (FileStream fs = File.OpenRead(path))
Box 2: AddFaceFromStreamAsync
Step 5 on
https://docs.microsoft.com/en-us/azure/cognitive-services/face/face-api-how-to-topics/how-to-add-faces