How do I add the code to CMSBased Template

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Hexa Extra Template

You can edit header.tpl file (header.tpl.new can be renamed to header.tpl in overrides folder). Overrides folder will help with keeping customization during updates.
The navbar li item can be duplicated and customized for your needs.

<a href="index.php?m=clientverification">client verification</a>

Version 4.5.x you don't need to edit any template you can do of the Hexa Extra Menu

Flattern Extra Template

You can edit header.tpl file (header.tpl.new can be renamed to header.tpl in overrides folder). Overrides folder will help with keeping customization during updates.
The navbar li item can be 

<li><a href="index.php?m=clientverification">client verification</a></li>