There are a lot of questions on this topic, but - they gave me no answer.
As from advices - there is one to set ServicePointManager.Expect100Continue = false
ServicePointManager.Expect100Continue = false
You can do it this way :
_request.ServicePoint.Expect100Continue = false;
This will disable the Expect: 100-continue for a particular HttpWebRequest instance.
Expect: 100-continue
HttpWebRequest