Cross-Domain GA4 Ads Attribution

I run a WordPress site that hands visitors off to a separate booking domain. The booking platform only lets me install GA4, and at the moment my Google Ads campaigns lose the original source once someone crosses over to that second domain. (comes in as "referral" source only vs ie paid search etc)

I need GA4 and Google Ads stitched together so every Product purchases conversion is attributed accurately, no matter which of the two domains it happens on. In practical terms this means:

• Configuring cross-domain measurement in GA4 for the WordPress domain and the third-party booking domain.
• Making sure the Google Ads gclid parameter is preserved through the redirect/iframe steps the booking engine uses.
• Sending the verified Product purchases event back into Google Ads as a primary conversion.
• Testing with real and debug traffic in Google Tag Manager (or another method you prefer) and showing that the original ad click is credited in both GA4 reports and the Google Ads interface.
• Supplying a concise setup note so I can replicate or adjust the tags later.

Everything is already upgraded to GA4; there is no Universal Analytics on the site. Let me know how soon you can implement and validate the tracking end-to-end.

(attached images/diagram illustrates)