You want to save a value in the report of the form, for instance customerid. But you do not want the respondent to see this. You can use a Hidden field.
Hidden field must be linked
A hidden field must be linked to a specific field in the database. See the animation below on how to link a field.
With this, you have the ability to add a value to the report, without a respondent seeing it.
Hidden field only works in a form within Mail+
A hidden field only works for forms that are on a landing page in Mail+. Only there can it retrieve the information from the database and recognize a contact. The hidden field feature does not work for external web pages.