[CRM 2016 Online] - Call external WebService - Crash Plugin execution and PluginRegistration tool debug.

Hi,

I've a Plugin on CRM 2016 Online and I need to call a external webService (Rest).

I can test and debug my webservice with browser, Postman, VS UnitTest and other... but,

when I call my webService from my Plugin Registration Tool in debug mode, this crash!!!!

Here my code:

using (var client = new WebClient())
{
    client.Headers.Add(HttpRequestHeader.Accept, "application/json");
    client.Headers.Add(HttpRequestHeader.ContentType, "application/json");
    try
    {
        response = client.UploadString(new Uri(string.Concat(_urlString, "/PUBLISH_PHONECALL_TOCTI")), "test");
    }
    catch (Exception ex)
    {
        //return message
    }
}

even though the call is in try-catch steatment, plugin registration tool crash.

Is there some limitation with Online sanbox execution?

And.... on production enviroment, have I to implement some authentication?