Cross-domain tracking is configured if multiple domains are in use

When marketing traffic lands on www.example.com and converts on app.example.com, missing cross-domain configuration resets the session at the subdomain boundary. The user who clicked your Google Ad and signed up is recorded as two separate visitors: one that bounced on the marketing site, one that appeared from (direct) / (none) on the app. Campaign ROI gets understated, paid channels look unprofitable, and budgets move away from the channels that actually work.

Configure the GA4 linker in your gtag config so both domains share a measurement cookie:

gtag('config', 'G-XXXXXXX', {
  linker: { domains: ['example.com', 'app.example.com'] }
})

In the GA4 UI, add both domains under Admin > Data Streams > Configure tag settings > Configure your domains. For Stripe checkout redirects, fire the conversion event on your post-redirect thank-you page, not inside Stripe's domain.