ok sorry for the last post, I finaly find the solution :
private function twitterRequest (URL : String):URLRequest{
var r:URLRequest = new URLRequest (URL);
var login64 : String = TwitterBase64.encode(_username + ":" +
_password);
var rArray : Array = new Array (new URLRequestHeader
("Authorization", "Basic " + login64));
var variables : URLVariables = new URLVariables ();
variables.authenticate = false;
r.data = variables;
r.requestHeaders = rArray;
r.method = URLRequestMethod.POST;
return r;