add_action('wp_head', function () {
  if (is_page('contact')) { // スラッグが contact の場合
    echo '<meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests">';
  }
});
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/totalfood/www/wordpress/wp-content/themes/totalfood/functions.php:26) in <b>/home/totalfood/www/wordpress/wp-includes/pluggable.php</b> on line <b>1296</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/totalfood/www/wordpress/wp-content/themes/totalfood/functions.php:26) in <b>/home/totalfood/www/wordpress/wp-includes/pluggable.php</b> on line <b>1299</b><br />