Hidden field in custom product form

I am trying to add a "hidden field" to a custom product form so I can send data with each order that will be visible only to the owner, and not to the user. Currently all fields in a "custom product form" are added to the confirmation email that the client receives.

I tried using a "hidden field" and changing the value with javascript, but this data was not actually sent anywhere as far as I can see. Not even in the downloadable order spreadsheet. It seems like the "hidden field" is only compatible with a "form block" currently, and not with a "custom product form"

Is there any way I can either hide certain form data from the confirmation email, or enable the usage of a hidden form on customized product? I am quite handy with code

Thanks in advance