My filebrowser can't update the txtUrl

I want to write a filebrowser for ckeditor myself. But I get into trouble with returning file url problem. I can't find a function to update the txtUrl, so that I have to write a function update the text myself. When click a file, it should update txtUrl automatically. But the id of txtUrl changed with different internet browsers, and the id seems to be random. And the below image isn't updated when the url is changed.

Any idea?