command.js

var url = document.URL;
javascript:(function(){
//window.twttr=window.twttr||{};
var D=550,A=450,C=screen.height,B=screen.width,H=Math.round((B/2)-(D/2)),G=0,F=document,E;
if(C>A){
  G=Math.round((C/2)-(A/2))
  }
  var win =window.open('http://twitter.com/share','','left='+H+',top='+G+',width='+D+',height='+A+',personalbar=0,toolbar=0,scrollbars=1,resizable=1');
   //var win = window.twtter.shareWin;
   win.focus();
    // If the window opened successfully (e.g: not blocked)
    if (win) {
        win.onload = function() {//onload can be blocked by the brower
         win.getElementById("status").value = url;
        };
    }

  E=F.createElement('script');E.src='http://platform.twitter.com/bookmarklets/share.js?v=1';
  F.getElementsByTagName('head')[0].appendChild(E)}());

command.json

{
  "name": "Share on Twitter with URL",
  "description": "A Backtick Hello World example command.",
  "icon": "http://icons.backtick.io.s3.amazonaws.com/backtick.png",
  "link": "http://backtick.io"
}

"name": "Example Command",
"description": "A Backtick Hello World example command.",
"icon": "http://icons.backtick.io.s3.amazonaws.com/backtick.png",
"link": "http://backtick.io"