How to send WebKitFormBoundary in multipart

[alpires2000]

Exist some way to send WebKitFormBoundary with MultiPartFormDataStream?

My code without WebKitFormBoundary:
...
          idhttp1.Request.ContentType := MultiPartFormDataStream.RequestContentType;
          MultiPartFormDataStream.AddFormField('field1', var);
          MultiPartFormDataStream.AddFile('file1', link_foto_envio,
            GetMimeTypeFromFile(link_foto_envio));
          MultiPartFormDataStream.Position := 0;
          pagina := idhttp1.Post
            ('https://upload.myserver.com/', MultiPartFormDataStream);

...

Some idea?