Cross-domain setup for Google and Microsoft Client Side

Hi There,

We have a hotel site that uses a booking engine for their purchases. The process works as follows: on our site, visitors click a “Book” button, which redirects them to the booking engine site where the actual booking takes place.

Unfortunately, we are not allowed to install GTM on the booking engine site. Instead, whenever we need to add something, such as a Microsoft Conversion or a Google Conversion for purchases, we provide them with code snippets which they then add to their backend.

The problem we’re encountering is that most of the conversions show up as (not set) even though we have proper UTM parameters in place. I am not an expert on managing multiple domains, so I am seeking guidance from experts. Is this issue occurring because we don’t have the same GTM container installed on both domains?

I verified the GA4 cross-domain tracking, and in GA4’s debug view, the device count does not change when moving from one domain to the other.

Could someone please guide me on how to resolve the (not set) issue? If the root cause is cross domain or the set up I explained doesn’t have to do with it?


If you are using server-side GA4 on your site and on booking site GA4 is reported directly to Google - that may be causing (not set). You should be reporting data from both sites one way (server-side) or another (client-side), do not use mixed approach, as client side an server side GA4 uses different sets of cookies.
Additionally, please double check whether cross-domain is properly configured in your Google tag settings.
You may also consider increasing session timeout, as by default in GA4 it is 30 minutes. If the conversion happens when the session that holds true traffic source has timed out - in your reports you may observe increased amount of direct or (not set) session source/medium simply because conversion happens when the new session started.