CRED form (toolset). Avoid WYSIWYG editor - use substitute

add_action('cred_save_data', 'my_save_content',10,2);
function my_save_content($post_id, $form_data)
{
    // if a specific form
    if ($form_data['id']==123)
    {
        if (isset($_POST['post_content_substitute']))
        {
            // add it to saved post meta
              $my_post = array(
                  'ID'           => $post_id,
                  'post_content' => $_POST['post_content_substitute']
              );

            // Update the post into the database
              wp_update_post( $my_post );
        }
    }
}