I've been trying many things and Finally I think that it works

  myClient = new Client(this, myserver, 80);
  myClient.write("POST /post.php  HTTP/1.0\r\n"); // Use the HTTP "GET" command to ask for a Web page
  myClient.write("Host: "+myserver+" \r\n");
  myClient.write("Content-Type: application/x-www-form-urlencoded\r\n");
  myClient.write("Content-Length: 32\r\n");
  myClient.write("\r\n");

  myClient.write("mode=login&user=test&password=test");
  myClient.write("\r\n");

I think that the tip is to write \r\n at the final of all lines, moreover It's important to write the correct number at Content-Length. Before I wrote \r\n the server delayed some seconds and returned a 408 error, but if I write a simple GET sentence without those \r\n it works. I'm new in http.