WordPress – Adding Link to Admin Sidebar

add_action( 'admin_menu', 'linked_url' );
function linked_url() {
add_menu_page( 'linked_url', 'External link', 'read', 'my_slug', '', 'dashicons-text', 1 );
}

add_action( 'admin_menu' , 'linkedurl_function' );
function linkedurl_function() {
global $menu;
$menu[1][2] = "http://www.example.com";
}