How did a tourism operator fix broken attribution on an external booking platform?

What was the problem?

A tourism operator sends visitors from its main website to an external booking platform to complete a booking. The booking platform runs on a different domain. Visitors arrived from ads on the main site, then crossed over to the booking domain to pay.

At that handoff, attribution broke. The booking happened, but it was not reported with the ad source that brought the visitor in. From the reporting point of view, the paid campaigns were doing the hard work and something else was getting the credit.

What did I build?

I built cross-domain attribution between the main website and the booking platform, so a visitor's session and ad source carry across the domain change. Bookings now report the correct ad source in both GA4 and Google Ads.

Why does attribution break across two domains?

Analytics and ad tools identify a visitor with first-party cookies, and cookies belong to one domain. When a visitor clicks from the main site to a booking page on a different domain, the booking site cannot read the cookies set on the main site. Without extra setup, it treats the visitor as brand new.

The usual symptoms are bookings credited to the main website as a referral, or to direct traffic, instead of the Google Ads or other campaign that started the journey. Google Ads may miss the booking conversion completely because the click information never reached the booking domain.

How does cross-domain attribution fix it?

1. 1

   Track both domains together

   The main site and the booking platform are measured as one journey, with the same analytics configuration on both sides.

2. 2

   Pass the visitor identity across the link

   When a visitor clicks through to book, a linker parameter carries the session and click information to the booking domain, where it is picked up.

3. 3

   Stop self-referrals

   The main website is prevented from appearing as the traffic source of its own bookings, so the original channel keeps the credit.

4. 4

   Fire the booking conversion with the right source

   The completed booking is recorded in GA4 and sent to Google Ads with the attribution that came across from the first domain.

What do you need from a booking platform to make this work?

The booking platform needs to let you add your own tracking, such as a Google Tag Manager container or GA4 tag, and it must keep URL parameters intact when visitors arrive. Some platforms run inside an iframe or strip parameters, which needs a different approach. Checking this early saves a lot of time.

How can you tell if your own cross-domain tracking is broken?

Look at where GA4 says your bookings come from. If a large share of booking conversions are credited to your own website as a referral, or to direct traffic, the visitor's original source is probably being lost at the domain change. Another sign is Google Ads showing far fewer booking conversions than the booking platform reports. You can also click through from your site to the booking page and check whether a linker parameter appears in the address bar.

What changed for the operator?

Bookings now report the ad source that actually brought the visitor in, in both GA4 and Google Ads. The operator can see which campaigns produce bookings, and Google Ads can optimize towards completed bookings rather than visits to the main site.