How do I track Conversion sources when my traffic goes between two sites?
I have a main site that most of my traffic arrives at. My visitors then are directed to a landing page on a different domain. To convert they either fill in a form on the landing page and when they have completed that form they are bounced back to a ‘thankyou for completing the form’ page on the main site OR they are navigated back to the main site before completing a form. These are my two conversions.
I use GA’s Acquisition/All Traffic/Source.Medium report to see how people visit either my main site or the landing page. The problem is that I need to know, of the people who convert, how they ORIGINALLY arrived at the main site before heading over to the landing page. At the moment that report shows that the vast majority of my conversions are referrals from the landing page.How can I track visitors from initial arrival on the main site TO landing page TO main site conversion and work out the original source of my conversions?
Many thanks for any help!
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The short answer is:
The most robust and cost-effective solution is implementing server-side tracking using Google Tag Manager (GTM) with Stape and leveraging the Google Analytics Measurement Protocol for sending conversion data directly from your main site’s server.
This approach effectively bypasses the typical client-side referral issues caused by the cross-domain navigation.
When the user first arrives at the main site, a unique User ID or Client ID should be captured and passed to the landing page domain, and then returned to the main site upon form completion or navigation.
The server-side GTM container on Stape receives the ‘Purchase’ or GenerateLead event from the main site and uses the stored original ID with the Google Analytics Measurement Protocol to accurately attribute the conversion to the original source/medium.
This method ensures data consistency, is less susceptible to ad blockers, and eliminates the misleading referral traffic by enforcing the correct attribution model directly on the server.
The long answer is:
Your core problem is one of cross-domain tracking failure and referral exclusion logic in a client-side context, which is causing the landing page domain to overwrite the true, original source in your Google Analytics 4 (GA4) reports.
The solution requires a shift to a server-side model for handling conversion data, which gives you granular control over the attribution logic.
The recommended architecture involves Google Tag Manager (GTM) on your main site to capture the initial GA4 client ID (cid), Stape as your server-side GTM environment, and the Google Analytics Measurement Protocol as the communication API.
When a user lands on the main site, GTM should capture the unique GA4 Client ID and then pass this ID as a URL parameter to the second domain’s landing page.
This is a crucial step that stitches the user journey across domains, which is superior to the automatic linker functionality when strict control is needed.
When the user either completes the form and is bounced back to the ‘thank you’ page on the main site or navigates back to the main site before converting, the main site’s server-side GTM endpoint must be activated.
Instead of relying on a client-side GA4 tag to fire on the ‘thank you’ page, the main site’s server should make a direct, backend call to your server-side GTM container on Stape, sending a PurchaseGenerateLead
This server-side GTM container on Stape then takes the conversion event and uses the captured original Client ID to construct a final payload to the Google Analytics Measurement Protocol.
By sending the hit with the original Client ID from the first visit, you force GA4 to attribute the conversion to the initial Source/Medium stored against that unique user identifier, completely bypassing the referral logic that is currently showing the landing page as the source.
This is a highly cost-effective solution because it reuses your existing GTM and GA4 infrastructure, adding a server environment like Stape, which offers better performance and control than building and maintaining custom API endpoints on Google Cloud Platform (GCP) or other cloud services.
For reporting and analysis, it is also highly recommended to integrate BigQuery API to push your raw GA4 event data into BigQuery, where you can perform custom, multi-site attribution modeling that is impossible within the standard GA4 interface, and then visualize these clean reports using the Looker Studio API for a complete, end-to-end data pipeline.
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