external_id
external_id is a field that carries an external click or user identifier, usually the clickid from a tracker, when a conversion is sent to an ad platform through a server-side API, such as Facebook's Conversions API (CAPI). It's what lets the platform match that conversion to the click that caused it, even without a browser pixel or cookies.
Why external_id matters in S2S tracking
Server-to-server conversion delivery, unlike pixel tracking, has no automatic cookie matching — the platform only gets what's explicitly passed in the request. external_id is one of the key fields in that request: it's what the platform uses, alongside other signals like fbclid, to tie a server-sent conversion back to the original ad click.
external_id across the PWA-tracker-offer chain
The usual flow: the ad network passes a click with its own ID, the tracker stores it as an internal click ID, and plugs it in as external_id when sending the conversion to CAPI once the offer fires a postback. APEX supports postback/S2S integrations (Keitaro↔FB CAPI and others) that pass external_id automatically, so the buyer doesn't have to hand-build requests to the ad platform's API.
FAQ
- Are external_id and clickid the same thing?
- Often it's literally the same value under two different names: clickid is the tracker's term, external_id is that same value in the format the ad platform expects for server-side conversions.
- What happens if external_id isn't sent to CAPI?
- Facebook (or another platform) can't reliably match the conversion to a specific click or ad, so some conversions fall out of attribution and never inform campaign optimization.
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