How to Track HubSpot Forms in GA4 Using Google Tag Manager

HubSpot forms don't talk to GA4 out of the box.

They load inside an iframe. GA4 can't see inside iframes. So unless you handle it yourself, submissions aren't tracked.

That means your conversion data is incomplete, and attribution is off.

Here's how to fix it.

Why This Needs a Workaround

GA4 can detect standard form submissions.

HubSpot doesn't use standard forms. It loads them inside an iframe, which is effectively a separate page.

GTM and GA4 can't access what happens inside that iframe.

HubSpot does send a message when a form is submitted.

You just need to catch it, push it into the data layer, and fire a GA4 event from there.

What You Need

• GA4 property (Measurement ID starting with G-)
• Google Tag Manager installed
• A HubSpot form you can test with
• Publish access in GTM

Step 1: Add the HubSpot Listener

Create a Custom HTML tag in GTM.

Paste this:

<script>
  // v3 forms (legacy embed)
  window.addEventListener("message", function(event) {
    if (
      event.data.type === 'hsFormCallback' &&
      event.data.eventName === 'onFormSubmitted'
    ) {
      window.dataLayer.push({
        event: 'hubspot-form-submit',
        'hs-form-guid': event.data.id
      });
    }
  });

  // v4 forms (new editor)
  window.addEventListener(
    "hs-form-event:on-submission:success",
    function(event) {
      var form =
        window.HubSpotFormsV4 &&
        window.HubSpotFormsV4.getFormFromEvent
          ? window.HubSpotFormsV4.getFormFromEvent(event)
          : null;

      var formId = form && form.getFormId
        ? form.getFormId()
        : (event.detail && event.detail.formId
          ? event.detail.formId
          : 'unknown');

      window.dataLayer.push({
        event: 'hubspot-form-submit',
        'hs-form-guid': formId
      });
    }
  );
</script>

Set the trigger to All Pages.

Name it something clear.

Step 2: Create the Trigger

Create a Custom Event trigger.

Event name:

hubspot-form-submit

Set to fire on all custom events.

Step 3: Capture the Form ID (optional, but useful)

Create a Data Layer Variable:

• Name: hs-form-guid

This lets you distinguish between forms later.

Step 4: Send the Event to GA4

Create a GA4 Event tag.

• Event name: form_submission (or generate_lead)
• Measurement ID: your GA4 ID

Add parameter:

• form_id → Data Layer Variable

Trigger it on your custom event.

Step 5: Test It

Use GTM preview.

Submit a form.

You should see:

1. hubspot-form-submit in the data layer
2. trigger fires
3. GA4 event fires

If nothing appears:

• listener not working (usually v3/v4 issue)

If trigger doesn't fire:

• event name mismatch

Step 6: Register the Parameter in GA4

Go to:

Admin → Custom definitions

Create:

• Name: Form ID
• Scope: Event
• Parameter: form_id

Without this, GA4 won't show the value.

Step 7: Mark as Key Event

If form submissions matter, mark the event as a key event.

Admin → Events → toggle on

This allows:

• conversion reporting
• attribution
• Google Ads optimisation

HubSpot's Native GA Integration

HubSpot can add GA4 tracking to its own pages.

That only helps if your forms live on HubSpot pages.

If your forms are embedded on your site, it doesn't solve this.

It also gives you less control over events and parameters.

Common Issues

No data layer event

Usually a v3/v4 mismatch or script not firing.

Event fires but nothing in GA4

Check DebugView first. Reports lag.

Multiple forms, no breakdown

You need both:

• data layer variable
• custom dimension

Numbers don't match HubSpot

They won't.

HubSpot = server-side. GA4 = client-side.

Ad blockers, consent, timing all affect GA4.

Use:

• HubSpot for leads
• GA4 for attribution

What This Actually Fixes

Without this, you don't know which channels are driving leads.

You just see traffic.

With it, you can see:

• which campaigns generate submissions
• which pages convert
• which forms actually get used

That's the difference between reporting activity and understanding what's working.