custom fields for tracking code

Hello,

I would like to include conversion tracking in my Contactform7.
To do this, I have to include this code in the custom fields:

<?php
$ga_script = get_post_meta($post->ID, 'tracking-code', true);
if ($ga_script) {
echo get_post_meta($post->ID, 'tracking-code', true);
} else {
//no script;
}
?>

However, these are not displayed on the page.
How can I activate these custom fields?

I found other topics, but I didn’t understand it.

Thanks in advance:

Greeting Heiko