When you are getting your ID (from web app)? If you can capture your ID when message sent you can add like bellow code
add_action( 'fep_action_message_after_send', function( $message_id, $message, $inserted_message ){
add_post_meta( $message_id, '_your_custom_meta_key', $id_from_web_app );
}, 10, 3 );