Can someone walk me through the dynamic fields

Hi all, I am trying to add the title of the ‘page’ the form is on to be shown in the subject field of the form when sent. I have tried to do some of the examples using for example

[dynamichidden pagetitle "CF7_get_post_var key='title'"]

By all I get is that shortcode being displayed.

Is there something else I need to add, like to the theme functions file?

Cheers