You can get the url from javascript on any of the inAppBrowser events (
loadstart, loadstop, loaderror, exit)var ref = cordova.InAppBrowser.open('http://apache.org', '_blank', 'location=yes');
var myCallback = function(event) { alert(event.url); }
ref.addEventListener('loadstart', myCallback);
But for the "bookmark" button, I think you will need to edit the inAppBrowser plugin to add a native button, and as you will use a native button and you will use native code, you can get the url with
webView.getOriginalUrl();
Other way might be to use executeScript and insertCSS to inject javascript and css to create a button inside the page you display