display link to a custom plugin in admin menu bar

if you want to add a top level menu page then you need to use add_menu_page() function more details on codex

add_action( 'admin_menu', 'register_my_custom_menu_page' );

function register_my_custom_menu_page(){
    add_menu_page( 'my plugin', 'plugin settings', 'manage_options', 'my-plugin-settings', 'my_plugin_custom_function', plugins_url( 'myplugin/images/icon.png' ), 66 );
}