To credit bookings to the website that sent the guest, add a utm_source parameter to every outbound link that points at your booking site. The value identifies the sending site. Use one stable lowercase slug per site, for example: https://yourbookingsite.com/?utm_source=partnersite
Put the per-site value in utm_source. The report credits each site based on the utm_source field. A fixed network-wide marker in a second parameter is harmless, but the system does not read it.
No whitelist is needed. You do not need to pre-register a list of sites. The tracker records whatever utm_source value arrives, and values it has not seen before appear automatically in the report’s unmatched-sources panel. Sites you add or drop later are covered without any configuration change. A site list only maps slugs to readable display names in the report, and you can update it at any time.
Redirects. Redirects are fine, as long as the final URL that lands on your booking site still carries the utm_source tag.
Attribution starts when tags go live. Attribution is forward-only from the moment your tags go live. Traffic that arrived before a link was tagged cannot be credited after the fact.