Add custom link to WP admin bar

add-link-to-wp-admin-bar.php

<?php
// Taken from:
// https://premium.wpmudev.org/blog/add-links-wordpress-admin-bar-toolbar/
// Notes added for posterity's sake

// Create function: this name add_toolbar_items will be used when we hook 
// into the admin bar code after the function is defined
function add_toolbar_items($admin_bar){
    // Creates parent link
    $admin_bar->add_menu( array(
        'id'    => 'my-item', // this can be anything you choose, make it describe the link you are adding
        'title' => 'My Item',
        'href'  => '/your-custom-url-or-slug-here',
        // Optional site_url() parameter for correct URL formatting in multisites with sub directory setups
        // 'href'  => site_url() . '/your-custom-url-or-slug-here',
        'meta'  => array(
            'title' => __('My Item'),            
        ),
    ));
    // Creates child link using the parent parameter, which you can see below
    $admin_bar->add_menu( array(
        'id'    => 'my-sub-item',
        'parent' => 'my-item', // this is used for creating a sub-item, must match the id of the parent item
        'title' => 'My Sub Menu Item',
        'href'  => '/your-custom-url-or-slug-here',
        'meta'  => array(
            'title' => __('My Sub Menu Item'),
            'target' => '_blank',
            'class' => 'my_menu_item_class'
        ),
    ));
    // Creates a second child link using the parent parameter, which you can see below
    $admin_bar->add_menu( array(
        'id'    => 'my-second-sub-item',
        'parent' => 'my-item', // this is used for creating a sub-item, must match the id of the parent item
        'title' => 'My Second Sub Menu Item',
        'href'  => '/your-custom-url-or-slug-here',
        'meta'  => array(
            'title' => __('My Second Sub Menu Item'),
            'target' => '_blank',
            'class' => 'my_menu_item_class'
        ),
    ));
}

// Action that calls our previously defined custom function called add_toolbar_items 
// in the admin_bar_menu hook that is built into WordPress 
add_action('admin_bar_menu', 'add_toolbar_items', 100);

This can be added to the functions.php, or split out into a separate file if that is your workflow.