/**
* Include CSS file for MyPlugin.
*/
function myplugin_scripts() {
wp_register_style( 'foo-styles', plugin_dir_url( __FILE__ ) . 'assets/foo-styles.css' );
wp_enqueue_style( 'foo-styles' );
}
add_action( 'wp_enqueue_scripts', 'myplugin_scripts' );
Answer
http://example.com/wp-content/plugins/my-plugin/assets/foo-styles.css