Drupal - add a link to admin menu bar

You can achieve using below step:

• Go to Administration -> Structure -> Menus
• Click on 'Add links' in 'Management' menu row
• Add 'Menu link title' and 'Path' for menu item.
• Set 'Parent link' for the menu item, so it would display in correct position.
• Save the form

Update:

By default, Management menu be only dispaly to administrator or the user which has permission to view it.

If you want to set any specific visibility permission, you can use Menu item visibility module.

This module exposes configurable and extendable visibility settings for menu links. For example, if you want to create a 'My account' link that points to /user, both anonymous and registered users have access to the /user path, so both will see the link, even if anonymous users do not technically have an account. Using this module will allow you to only have your 'My account' link visible to registered users, and hide it from anonymous users.