Adding a admin menu item that links to a URL

$wp_admin_bar->add_node() should be able to do what you want: https://codex.www.ads-software.com/Function_Reference/add_node

Specifically, you can take a look at this example from that page:

add_action( 'admin_bar_menu', 'toolbar_link_to_mypage', 999 );

function toolbar_link_to_mypage( $wp_admin_bar ) {
	$args = array(
		'id'    => 'my_page',
		'title' => 'My Page',
		'href'  => 'https://mysite.com/my-page/',
		'meta'  => array( 'class' => 'my-toolbar-page' )
	);
	$wp_admin_bar->add_node( $args );
}

Thanks for the help, OldRiver. This is sort of what I want. But instead of adding a link to the admin bar, I want to add it to the admin menu, the vertical menu on the left-hand side in the wordpress backend. Apologies for not explaining it correctly.

You were on the right track with Administration Menus, but you needed to dig a bit deeper. See Function_Reference/add_menu_page. In the parameters section you can see if you provide a $menu_slug argument that is a PHP file and omit the $function parameter, that PHP page is displayed when the menu option is selected. Many of the default admin panels work this way. Sub-menus have a similar setup.

bcworkz, thanks! I overlooked that part because I was looking for something like a URL parameter instead of a ‘slug’ parameter. But this will work just fine for me. Thanks!