# _atid

> The URL parameter that carries the resolved Click ID value, so platforms, form builders, and scripts the Tracking Tag cannot modify can still collect and store the click ID.

**_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](/glossary/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

1. **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.
2. **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](/glossary/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.