Can you please let me know how I can add a custom Tag button into WP Editor in
Text(HTML) mode like
what I would like to insert into the Post is something like
<hr class="dark-line-2" />
every time that user pushed on
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You could try a plugin like Visual Editor Custom Buttons.
Or you may want to do something custom. There's a solid example at http://code.tutsplus.com/tutorials/guide-to-creating-your-own-wordpress-editor-buttons--wp-30182.