You should use the following JS API on your custom web form to set identity to the visitors, just like the web forms on Zoho CRM normally do.

Please note that you should invoke the following JS API by passing the name and email address when the web form is being 'submitted.'

$zoho.salesiq.visitor.name(" < NAME > "); 

$zoho.salesiq.visitor.email(" < EMAIL > "); 

To know more about the SalesIQ JS APIs, click here:  https://www.zoho.com/salesiq/help/jsvisitorapi.html