_atid

_atid is a URL query parameter whose value is the resolved click ID. It exists for one reason: some surfaces can collect and store URL parameters but cannot be touched by the Tracking Tag — JavaScript form widgets, iframe embeds, forms injected after page load, and platforms without hidden form fields. Putting the click ID into the page URL lets those surfaces collect it themselves.

How _atid gets into the URL

Platform auto-append — on platforms that store URL parameters but have no hidden fields (Systeme.io and similar), AnyTrack appends _atid automatically.
AnyTrack('atclidToUrl') — a Tag command that appends the already-resolved click ID value to the landing-page URL. It is not token substitution and not a single-page-application feature (it can apply to SPAs, but that is not its purpose).

_atid and the two click-ID fill paths for forms

Tag substitution — a hidden field with default value --CLICK-ID-- is filled by the Tracking Tag automatically, without any _atid in the URL. This works only in form DOM the Tag can modify, and only during its load-time scan — forms injected later are never rescanned.
URL-param collection — a form the Tag cannot touch will never get --CLICK-ID-- substituted. Instead, get _atid into the page URL and name the collecting field _atid: the form builder itself fills the field from the URL parameter.

An unfilled --CLICK-ID-- placeholder on a JavaScript widget form is expected behavior for path 1 — the fix is path 2, not reinstalling the Tag.

What _atid is not

_atid is not a cross-domain parameter. Cross-domain tracking passes the click ID to a destination domain with atrefid, added to links at click time. And the Tag never substitutes --CLICK-ID-- in the loaded page’s own URL — a token placed in an ad’s final URL arrives literal, because no Tag ran before the landing page loaded.

Frequently asked questions

FAQ was last reviewed on 2026-08-01

I see _atid on my links and URLs — is something broken?

No. Seeing _atid is always expected. It is auto-appended on platforms that can collect URL parameters but have no hidden form fields (such as Systeme.io), and it has nothing to do with cross-domain tracking.

My hidden form field with --CLICK-ID-- is not filling. Do I need _atid in the URL?

Those are two independent fill paths. The Tag substitutes --CLICK-ID-- in form DOM it can modify, with no _atid needed. If the form is a JavaScript widget, iframe, or is injected after page load, the Tag cannot fill it — get _atid into the page URL (platform auto-append or AnyTrack('atclidToUrl')) and name the collecting field _atid so the form builder fills it from the URL.

My form builder rejects field names starting with an underscore.

The parameter name is customizable: AnyTrack('atclidToUrl', 'click_id') appends the value under the name you choose, and the form field maps to that name instead.