Insert Message before content of after title

You need to use ‘the_content‘ filter

function add_before_content($content) {
 return 'Before content area '.$content;
}
add_filter('the_content', add_before_content);

It will still be inside the content div. If you want it outside the div you have to edit related template files i.e. single.php or templates/content-single.php. It all depends on which pages you want to apply it.