How to track form submissions and leads back to their source

GA4 can tell you that 40 forms were submitted from Google Ads last month. It cannot tell you which of those 40 were the three that became customers, because GA4 does not hold your sales outcome. The fix is to store the source on the lead itself.

The do-it-yourself method

1. Add hidden fields to every form: utm_source, utm_medium, utm_campaign, utm_term, utm_content, gclid and the landing page.
2. Use a small script to read these from the URL on arrival and store them in first-party storage, so they survive the visitor clicking around before converting.
3. Fill the hidden fields from storage when the form loads.
4. Map the fields to your CRM so each lead carries its source.
5. Fire a GA4 event (generate_lead) on a successful submission and mark it as a key event.

This works, and costs nothing beyond time. It breaks quietly when a developer rebuilds a form, when a page loses its script, or when a visitor arrives without UTMs. Check it monthly.

When a platform is worth it

A lead tracking platform such as WhatConverts captures forms, calls and chats with their source automatically, then lets you mark leads as qualified or quoted and attach a value. Reports then show which campaigns produced revenue, not just submissions. Agencies use the same data to show clients lead quality, which tends to end the "but are these good leads?" conversation.