External.cs

public class ExternalOrderIntegration
{
    private readonly IMessageSession _messageSession;
    private readonly HttpClient _httpClient;

    public ExternalOrderIntegration(IMessageSession messageSession, HttpClient httpClient)
    {
        _messageSession = messageSession;
        _httpClient = httpClient;
    }

    public async Task PlaceOrder()
    {
        var response = await _httpClient.GetAsync("http://external.com/newOrders");
        var externalOrder = await response.Deserialize<ExternalOrder>();

        var placeOrder = new PlaceOrder
        {
            OrderId = externalOrder!.Id,
            // Translation to convert an external order or PlaceOrder command
        };
        await _messageSession.Send(placeOrder);
    }
}