standalone-form-shortcode.php

<?php
/**
 * Plugin Name: JS Widgets: Standalone Form Shortcode
 */

add_action( 'wp_enqueue_scripts', function() {
	wp_enqueue_style( 'js-widget-form' );
} );

add_shortcode( 'js-widget-standalone-form', function( $atts ) {

	global $wp_widget_factory;
	if ( empty( $atts['id_base'] ) ) {
		return;
	}

	$widget = null;
	foreach ( $wp_widget_factory->widgets as $widget_instance ) {
		if ( $atts['id_base'] === $widget_instance->id_base ) {
			$widget = $widget_instance;
			break;
		}
	}
	if ( ! $widget || ! ( $widget instanceof WP_JS_Widget ) ) {
		return;
	}

	ob_start();

	$widget->enqueue_control_scripts();
	$widget->render_form_template_scripts();

	$id = (string) rand();
	?>
	<div id="<?php echo esc_attr( $id ) ?>"></div>
	<script>
	jQuery( function( $ ) {
		var container = $( <?php echo wp_json_encode( '#' . $id ) ?> );
		var idBase = <?php echo wp_json_encode( $widget->id_base ) ?>;
		var form = new wp.widgets.formConstructor[ idBase ]( {
			model: new wp.customize.Value( { title: 'Hello world', text: 'Foood', filter: true } ),
			container: container
		} );
		form.render();
	} );
	</script>
	<?php

	return ob_get_clean();
} );