User ID — Cross-device user identification

What is the User ID add-on?

The User ID add-on automatically adds an X-DataNostro-User-ID header to all requests if a User ID is available in the request (from a cookie, query param, or POST data).

Why use User ID?

• Cross-device tracking: Link the same user across PC, mobile, tablet.
• Logged-in users: Track logged-in users across sessions.
• Enhanced Conversions: Send a hashed User ID to Google Ads/Meta.
• CRM sync: Link web tracking with CRM data.

How to activate

1. Go to Dashboard → Add-ons
2. Turn on the "User ID" toggle
3. The power-up automatically detects the User ID from:

User ID sources

The power-up looks for the User ID in this order:

1. The uid query parameter (e.g. ?uid=user123)
2. The user_id cookie
3. The _ga_uid cookie (custom)
4. GA4 user_id from the measurement protocol POST data
5. The GTM DataLayer user_id variable

The added header

X-DataNostro-User-ID: user_abc123xyz

Integration with the Custom Loader

If you have the Custom Loader active, you can set a user_identifier field that's automatically inserted into the GTM DataLayer:

window.dataLayer = window.dataLayer || [];
window.dataLayer.push({
  'user_id': '{{ user_identifier }}',
  'event': 'user_identified'
});

Enhanced Conversions with User ID

In the GTM server-side Google Ads tag use the User ID for Enhanced Conversions:

const userId = getRequestHeader('X-DataNostro-User-ID');
if (userId) {
  // Send hashed User ID to Google Ads
  enhancedConversionData.user_data = {
    hashed_user_id: sha256(userId)
  };
}