How you do this could be more or less hard depending on where the content is supposed to be included in the page, and you don’t explain that, but assuming you are including it in the post body…
add_filter(
'the_content',
function($content) {
global $post;
$db_showmap_options = get_option( 'db_showmap_options' );
if (!empty($db_showmap_options['page_id']) && $post->ID == $db_showmap_options['page_id']) {
$content = "whatever you need to do".$content;
}
return $content;
}
);
Untested but I am fairly sure that will work.
You may need output buffering, depending on how your showmap.php is written.