How does Hidden Fields work?

Assume you receive 30 new leads after your webform goes Live 15 of them signed up through the email campaign you sent, and the other fifteen came in through Facebook Lead ads. Thus, there are two sources for your Leads - your Email Campaign and Facebook.

Every time a lead opens your campaign, the URL parameter source is assigned the value email campaign in the backend in the first case. Similarly, source=facebook in the second case.

So you get clarity on which channels are generating the most Leads.