Pass a value via variable in Contact Form 7

snippet.php

// see Source
// http://blog.ostryweb.cz/2017/05/pass-values-from-shortcode-in-contact.html

// in Contact Form 7 Admin
[text my-extra-field default:shortcode_attr readonly]

// in Wordpress Template
<?php
$my_extra_field = "This can be dynamic";
$shortcode = '[contact-form-7 id="123" title="Name of the form" my-extra-field="'.$my_extra_field'"]';
echo do_shortcode($shortcode);
?>

// in functions.php
<?php
add_filter( 'shortcode_atts_wpcf7', 'custom_shortcode_atts_wpcf7_filter', 10, 3 );
function custom_shortcode_atts_wpcf7_filter( $out, $pairs, $atts ) {
    $my_attr = 'my_extra_field';
    if ( isset( $atts[$my_attr] ) ) {
        $out[$my_attr] = $atts[$my_attr];
    }
    return $out;
}
?>