Post request with parameters

Hello,

I have a problem to send a post request with parameters - this is my code :

WebServiceURL := 'https://...';
Request := Request.Create(WebServiceURL);
Request.Method := 'POST';
Request.KeepAlive := TRUE;
Request.Timeout := 30000;
Request.Accept('application/json');
Request.ContentType('multipart/form-data');
postString := 'param1=123&param2=456';
Request.ContentLength := STRLEN(postString); 
StreamWriter := StreamWriter.StreamWriter(Request.GetRequestStream);
StreamWriter.Write(postString);
StreamWriter.Close;

I get a 500 error (see picture) so I don't know anything about why its rejected
But if there is something that seems to be wrong to you, please help !

Bye