[hidden my-form-number default:shortcode_attr]
[contact-form-7 id="1" title="Form Title" my-form-number="my_unique_value"]
add_filter( 'shortcode_atts_wpcf7', 'custom_shortcode_atts_wpcf7_filter', 10, 3 );
function custom_shortcode_atts_wpcf7_filter( $out, $pairs, $atts ) {
$my_attr = 'my-form-number';
if ( isset( $atts[$my_attr] ) ) {
$out[$my_attr] = $atts[$my_attr];
}
return $out;
}