CDO records mapping to Contacts using a custom field

Our Eloqua instance is configured to use a custom ID field (which is a numeric field) as the unique value instead of the standard email address field.

We want to create a CDO that will automatically map the CDO records to the Contact list using the custom ID field. There is the option within the CDO to select the "Default Entity Mapping used in upload" which we set up to be the ID. This works well when we upload records to the CDO manually but it does not work when a record is added through a form for example. The records added through a form are not mapped to the Contact list.