Why This Matters

✅ More optimized ad delivery and conversions

How It Works

This helps Meta identify the same user session and attribute conversions accurately.

Requirements

Before setting this up, make sure:

• Both domains share the same GTM container ID

• Meta Pixel is installed on both sites (via GTM or directly)

Step-by-Step Implementation

Step 1: Append fbclid/_fbc/_fbp to Outbound Links (Site A)

• Tag Type: Create A new Custom HTML
• Trigger: All Pages (Safe - Script waits for DOM automatically). 

Replace siteB.com with your target domain.

This script reads tracking parameters (fbclid, _fbc, _fbp) from cookies and appends them to outbound links leading to your target domain(s)

<script>
(function () {
  // === CONFIG ===
  var TARGET_DOMAIN = 'siteB.com'; // <-- Change your destination domain

  
  var KEYS = ['fbclid', '_fbc', '_fbp'];

  function getCookie(name) {
    var m = document.cookie.match(new RegExp('(?:^|; )' + name + '=([^;]*)'));
    return m ? decodeURIComponent(m[1]) : '';
  }

  function setQueryParam(href, key, value) {
    if (!value) return href;

    var hash = '';
    var hashIndex = href.indexOf('#');
    if (hashIndex > -1) {
      hash = href.substring(hashIndex);
      href = href.substring(0, hashIndex);
    }

    var qIndex = href.indexOf('?');
    var base = href;
    var qs = '';
    if (qIndex > -1) {
      base = href.substring(0, qIndex);
      qs = href.substring(qIndex + 1);
    }

    var parts = qs ? qs.split('&') : [];
    var found = false;
    for (var i = 0; i < parts.length; i++) {
      var kv = parts[i].split('=');
      if (decodeURIComponent(kv[0]) === key) {
        parts[i] = encodeURIComponent(key) + '=' + encodeURIComponent(value);
        found = true;
        break;
      }
    }
    if (!found) parts.push(encodeURIComponent(key) + '=' + encodeURIComponent(value));

    var newQs = [];
    for (var j = 0; j < parts.length; j++) {
      if (parts[j]) newQs.push(parts[j]);
    }
    return base + (newQs.length ? ('?' + newQs.join('&')) : '') + hash;
  }

  function rewriteLinks() {
    var links = document.getElementsByTagName('a');
    for (var i = 0; i < links.length; i++) {
      var a = links[i];

      // hostname resolve (anchor trick)
      var parser = document.createElement('a');
      parser.href = a.href;
      var host = parser.hostname || '';

      if (host.indexOf(TARGET_DOMAIN) > -1) {
        for (var k = 0; k < KEYS.length; k++) {
          var val = getCookie(KEYS[k]);
          if (val) {
            a.href = setQueryParam(a.href, KEYS[k], val);
          }
        }
      }
    }
  }

  if (document.readyState === 'loading') {
    document.addEventListener('DOMContentLoaded', rewriteLinks);
  } else {
    rewriteLinks();
  }
})();
</script>

Step 2: Capture Params and Set Cookies (Site B)

• Tag Type: Create a new Custom HTML
• Trigger: All Pages

This script checks if fbclid, _fbc, or _fbp parameters exist in the URL and stores them in first-party cookies so Meta Pixel can access them across domains.

<script>
(function () {
  var KEYS = ['fbclid', '_fbc', '_fbp'];

  function getQueryParam(name) {
    var regex = new RegExp('[?&]' + name + '=([^&]+)');
    var m = window.location.search.match(regex);
    return m ? m[1] : '';
  }

  function setCookie(name, value, days, domain) {
    if (!value) return;
    var cookie = name + '=' + value + ';path=/';
    if (domain) cookie += ';domain=' + domain;
    if (days) cookie += ';max-age=' + (days * 24 * 60 * 60);
    document.cookie = cookie;
  }

  function getRootDomain() {
    var host = window.location.hostname.split('.');
    if (host.length >= 2) {
      return '.' + host.slice(-2).join('.');
    }
    return window.location.hostname;
  }

  var rootDomain = getRootDomain();
  for (var i = 0; i < KEYS.length; i++) {
    var key = KEYS[i];
    var val = getQueryParam(key);
    if (val) setCookie(key, val, 90, rootDomain);
  }
})();
</script>

Step 3: Verify Tracking

If you’re testing using Meta Pixel Helper, open the Network tab and confirm the _fbc parameter value in the event payload is identical on both domains.

Outcome

After implementing this setup:

• Users moving between domains retain their original Meta Click ID (_fbc), ensuring consistent attribution.

• Conversions properly attribute back to the ad click.

• You’ll see improved event matching and conversion accuracy in Meta Ads Manager.

• Events show consistent fbc parameter across both domains (verify via Network tab)

Bonus: Server-Side Tagging

When sending events server-side, make sure to forward the same _fbc and _fbp values from your client cookies into your Meta Conversion API (CAPI) requests for proper deduplication.

Conclusion

Cross-domain tracking is essential for accurate Meta attribution in multi-domain user journeys.

By passing and preserving the _fbc parameter between domains, you ensure Meta Ads can correctly identify users and measure conversions.

This method is simple, compliant, and future-proof, and can be expanded easily into server-side setups for even cleaner data.