FeralReason on "Add a Custom Mail Tag to Contact Form 7"

I have landing pages with links to a Contact Form 7 form page. When a user clicks the link, I capture the landing page URL and write it to a cookie called "landingPage" (using an on-click event). What I want is add that landing page URL to the message body so my client knows which page the user clicked from. My preference is to add a special mail tag called '_landingpage' that reads the cookie and inserts it in the email message body.

I can hack the Contact Form 7 plugin to do this by adding the following code in the mail.php wpcf7_special_mail_tag() function:

if ('_landingpage' == $name) {
      $cookie_name = "landingPage";
      if(!isset($_COOKIE[$cookie_name])) {
          return 'Original page not retrievable.';
      } else {
          return  $_COOKIE[$cookie_name];
      }
  }

But, since this will be overwritten with a plugin update, I should create a function in functions.php (or a custom plugin) to do this. I thought the hook might be: 'wpcf7_before_send_mail'. But I can't seem to figure out how to add a mail tag -- or even overwrite an existing mail tag with the cookie value to use that.

// Failed attempt to overwrite the '_url' mail tag value with my landingPage URL:

function URL_of_form_link( $cf7 )
{
  $cookie_name = "landingPage";
  if(!isset($_COOKIE[$cookie_name])) {
      $cf7->posted_data["_url"] = 'Original page not retrievable.';
  } else {
      $cf7->posted_data["_url"] = $_COOKIE[$cookie_name];
  }
  // ... no permutation I tried worked
}
add_action( 'wpcf7_before_send_mail', 'URL_of_form_link' );

Does anyone have an idea of how to do this?

https://wordpress.org/plugins/contact-form-7/