usingsession
--- a/src/EventSubscriber/PopupMessageSubscriber.php
+++ b/src/EventSubscriber/PopupMessageSubscriber.php
        ($message_body_variable['format'] ?? filter_default_format())
      );

+      $render_array = [
+        '#theme' => 'popup_message',
+        '#title' => $message_title,
+        '#body' => $message_body,
+      ];
+      $markup = $this->renderer->renderPlain($render_array);