umbraco forms add extra data to saved entry

Hi,

We need add extra data to each saved form entry. We must collect tracking data from our affiliates.

I can create hidden form fields thru C# and populate their values from JS.

But how do i add the data to each saved form entry, without having to add the field types on each created form in the backoffice?

I tried looking into RecordSavingNotification, but cant figure out how to use this.