Add text in "add post" page

I’m afraid it’s not that simple. You must never edit core WordPress files. Things will go badly for you down the line if you do this. Changing the functionality of WordPress is typically done by hooking into various actions and filters to change how things work.

From the sounds of things and what I know of the tags widget, you will probably be better off inserting your custom text by using javascript or jQuery to do the job. Do not try to hardcode script blocks or meta links to invoke javascript. Instead, use wp_enqueue_script(), the meta links will be inserted for you and any dependencies are resolved.