WinHttpRequest: Send method

Question

I'm trying to pass parameters in the request body, the documentation says :

The request to be sent was defined in a prior call to the Open method. The calling application can provide data to be sent to the server through the Body parameter. If the HTTP verb of the object's Open is "GET", this method sends the request without Body, even if it is provided by the calling application.

So, I need to use POST with body. But when I use use POST with body I have error "Bad Request: message text is empty" and when I use GET with body result is ok. Why?

My code:

WinHttp = NEW COMObject("WinHttp.WinHttpRequest.5.1"); 
WinHttp.Open("GET", "http://api.telegram.org/botbotname/sendMessage", 0);

WinHttp.setRequestHeader("Content-type", "application/json");

JSONWr = New JSONWriter();  
JSONWr.ValidateStructure = False;   

JSONParams = New JSONWriterSettings( , Chars.Tab);  

JSONWr.SetString(JSONParams);   

JSONWr.WriteStartObject();

JSONWr.WritePropertyName("chat_id");
JSONWr.WriteValue(UserId);

JSONWr.WritePropertyName("text");
JSONWr.WriteValue(Text);

JSONWr.WriteEndObject();

JSONString = JSONWr.Close();

WinHttp.Send(JSONString);

work, but how? And why the same with POST doesn`t work?