The short answer is that tracking form submissions inside an iframe is generally quite difficult because of security restrictions (the same-origin policy), as the parent page (your landing page) can’t easily access the contents of the iframe (the Brevo form) unless the Brevo form is on the same domain or they’ve specifically implemented a way to pass messages back, which is unlikely for a standard embedded form.
Because of this limitation, the cleaner approach is often to avoid the iframe and embed the form’s HTML directly if Brevo allows it, which would let you track the button click or the form submission event using Google Tag Manager (GTM) more easily via a Custom Event or a Form Submission trigger, or by tracking the button’s ID or class.
Even if you switch to direct HTML, a more robust and future-proof strategy, especially with the shifts toward server-side tracking and privacy updates, involves using a server-side setup.
While setting up client-side tracking with GTM for the Send button click is a good start to link to Google Ads, the most resilient way to track a conversion from a third-party form like Brevo, and to ensure you capture conversions even when client-side tracking is blocked, is to use a server-side solution.
This involves sending the conversion data directly from Brevo’s system to Google’s systems.
You could leverage the Brevo API to detect a successful form submission, and then use that event to trigger a conversion call directly to the Google Ads API and/or the Google Analytics Data API.
This method ensures the conversion is recorded accurately server-to-server.
The full stack would involve: Brevo API (to confirm the submission) + Google Ads API (to log the Purchase or Lead conversion directly) + Google Analytics Data API (to send a Measurement Protocol event for a corresponding purchase or lead) + GTM (for managing client-side tracking you still want) + a service like Stape or Google Cloud Platform (GCP).
GTM, potentially running server-side on Stape or GCP, would act as the conduit, receiving the submission success from Brevo (via a webhook or similar) and forwarding it to Google Ads and Analytics.
This setup is more complex but is the most reliable way to report conversions, bypasses many client-side tracking limitations, and gives you complete control over the data being sent to Google’s platforms, using powerful Standard Events like Lead or Purchase instead of relying solely on button clicks which can sometimes fire even if the form submission ultimately fails.
This approach allows you to confidently attribute the conversions in Google Ads because it’s based on a confirmed submission, not just a successful button click.