How to add image or copy on click BN ID?

You can write any item that exists in the database and it automatically will link it like this: Io, Nef, Scroll of town portal, Tyrael
(if you're not sure how it's written first search it in the top right corner in "Search the database")

For Battlenet, this happens automatically in trades, all you have to do is go to your profile, Edit profile and add your tag under the "Battle Net ID" box.