①Contact Form7で以下を設定

[text post_title]

②functions.phpに以下を追記

function set_post_title_to_cf7_tag( $tag ){
    if ( ! is_array( $tag ) ){
        return $tag;    
    }
 
    $name = $tag['name'];
    if( $name == 'post_title' ){
        $res_get_the_title = get_the_title();
        $tag['values'] = (array)$res_get_the_title;
    }
    return $tag;
}
add_filter( 'wpcf7_form_tag', 'set_post_title_to_cf7_tag', 11 );