setTemplateData example

'recipients' => ['baz@example.com' => 'Baz'],
            'salesChannelId' => TestDefaults::SALES_CHANNEL,
            'contentHtml' => '<h1>Test</h1>',
            'contentPlain' => 'Test',
            'subject' => 'Test subject & content',
        ];

        $this->addEventListener(
            $this->getContainer()->get('event_dispatcher'),
            MailBeforeValidateEvent::class,
            function DMailBeforeValidateEvent $event): void {
                $event->setTemplateData(
                    [...$event->getTemplateData(), ...['plugin-value' => true]]
                );
            }
        );

        $mailService->send($data, Context::createDefaultContext());

        static::assertArrayHasKey(0, $environment->getCalls());
        $first = $environment->getCalls()[0];
        static::assertArrayHasKey('data', $first);
        static::assertArrayHasKey('plugin-value', $first['data']);
    }