Frontend submission upload image via external url

For upload from external url you should use media_sideload_image function. I assume $posterurl is url of external image, after wp_insert_post add below code in if($movie_id){} block. It will upload image from external url to this post and if you want to set this image as post feature image use set_post_thumbnail otherwiase comment that line. and save a image url as post meta field in poster_path meta key.

# $posterurl third party url
if(!empty($posterurl)) {
  require_once(ABSPATH . 'wp-admin/includes/media.php');
  require_once(ABSPATH . 'wp-admin/includes/file.php');
  require_once(ABSPATH . 'wp-admin/includes/image.php');

  $attach_id = media_sideload_image($posterurl,$movie_id,'','id');
  # set post thumbnail for post
  set_post_thumbnail( $movie_id, $attach_id );
  # save meta field for poster path
  update_post_meta($movie_id, 'poster_path', esc_attr(strip_tags($posterurl)));
}