Can't define BackgroundSocketProcessor with ASP.NET 8

Question

I'm following the instruction by Microsoft but I can't use BackgroundSocketProcessor . Do I miss any library ? Program.cs:

app.Run(async (context) =>
{
    using var webSocket = await context.WebSockets.AcceptWebSocketAsync();
    var socketFinishedTcs = new TaskCompletionSource<object>();

    BackgroundSocketProcessor.AddSocket(webSocket, socketFinishedTcs);

    await socketFinishedTcs.Task;
});

I'm really confused and trying to figure out why it doesn't work