procedure TForm2.Button3Click(Sender: TObject);
var
  LoginInfo: TStringList;
  Response: TStringStream;
begin
  Memo1.Clear;
  try
    LoginInfo := TStringList.Create;
    Response := TStringStream.Create('');
    LoginInfo.Add('Data()');
    IdHTTP1.Request.ContentType := 'text/plain';
    IdHTTP1.POST('http://192.168.100.100:5555/command',LoginInfo, Response);
    Memo1.Lines.Add(Response.DataString);
  finally
    begin
      Response.Free;
      LoginInfo.Free;
    end;
  end;
end;

POST /command HTTP/1.0
Connection: keep-alive
Content-Type: text/plain; charset=ISO-8859-1
Content-Length: 10
Host: 192.168.100.100:5555
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding: identity
User-Agent: Mozilla/3.0 (compatible; Indy Library)

Data%28%29

Transmission Control Protocol, Src Port: 56039, Dst Port: 55400, Seq: 1, Ack: 1, Len: 122
Hypertext Transfer Protocol
    POST %2Fcommand HTTP/1.1\r\n
        [Expert Info (Chat/Sequence): POST %2Fcommand HTTP/1.1\r\n]
        Request Method: POST
        Request URI: %2Fcommand
        Request Version: HTTP/1.1
    Connection: Keep-Alive\r\n
    Content-Type: text/plain\r\n
    Content-Length: 10\r\n
        [Content length: 10]
    \r\n
    [HTTP request 1/131]
    [Response in frame: 66]
    [Next request in frame: 129]
    File Data: 10 bytes
Line-based text data: text/plain (1 lines)
    Data()