Hidden fields in mail

Hi there,

I am using a calculator (https://wordpress.org/support/plugin/cf7-calculator/) and I want to hide the outcome with the hidden snippet (https://contactform7.com/hidden-field/) because I want the outcome only to be made known in an (auto-respond) e-mail.

However when I hide the calculator snippet, it isn’t sent in the mail. When I un-hide it, it is shown in the email.

Can you explain me how I can hide the outcome snippet so that it isn’t visible on the form itself, but is only sent in the (auto-respond) e-mail?