HubSpot's collected forms tool allows you to capture form submissions from non-HubSpot forms on any website that has the HubSpot tracking code installed and running.
The collected forms tool is able to identify your website's external forms based on the
<form> tag in the HTML code. As soon as a visitor submits a form, HubSpot stores the submission information in a cookie on the visitor's browser. This cookie is called li_formsub. The form submission data stored in this cookie is then pushed back into HubSpot to either create a new contact or update any existing contact record with the same email address. You can learn more about how browser cookies work in this post from the HubSpot User Blog.
HubSpot will try to map form fields to the following default properties: Email, First Name, Last Name, and Phone Number. Other fields included on your forms will be created as new HubSpot properties in order to store those values.
A submission notification email will be sent (if enabled) and the contact will be pushed to any connected email provider lists. This process can take approximately two minutes.