Hi,
Please take a look at my sample code
add_action( 'rwmb_frontend_after_save_post', function( $object_post ) {
add_post_meta( $object_post->post_id, 'test_post_meta', 'Test Post Meta Value' );
} );
You can use 4 properties of the MB Frontend Post class $post_type
$post_id
$fields
$config
.
For more information, please follow the documentation
https://docs.metabox.io/extensions/mb-frontend-submission/#post-actions