private Subject<string> MessageReceived = new Subject<string>();

//this method gets called every time a message is received from socket
internal void OnReceiveMessage(string message)
{
    MessageReceived.OnNext(message);
    ProcessMessage(message);
}

public async Task<string> TestMethod()
{
    var expectedMessage = MessageReceived.Where(x => x.EndsWith("D") && x.EndsWith("F")).FirstOrDefaultAsync();
    await SendMessage("ABC");
    return await expectedMessage;
}