By default, HubSpot only creates or updates a contact based on the default Email field, and will not create a contact when they fill out a custom email field separate from the default Email field.
If you allow submissions to create a contact without an email address, these contacts will be created but it is recommended to use the default Email field as the field is referenced by other tools in HubSpot, such as workflows and marketing emails. These tools will not work with a custom email field.