command.js

(function () {
    var s = document.createElement("script");
    s.setAttribute("id", "bitmark_js");
    s.setAttribute("type", "text/javascript");
    s.setAttribute("src", "//bitly.com/a/bitmarklet.js");
    (top.document.body || top.document.getElementsByTagName("head")[0]).appendChild(s);
})();

command.json

{
  "name": "Bitly Bitmark",
  "description": "Create a bitly bitmark from the current page.",
  "icon": "https://backtickio.s3.amazonaws.com/icons/bitly.png",
  "link": "http://bit.ly"
}