Upload featured image on custom post from WordPress front-end

wp-featured-image-upload-for-front-end.php

<?php
//Just uploading photo or attachments
require_once(ABSPATH . "wp-admin" . '/includes/image.php');
require_once(ABSPATH . "wp-admin" . '/includes/file.php');
require_once(ABSPATH . "wp-admin" . '/includes/media.php');
         

$file_handler = 'upload_attachment' //Form attachment Field name.
$attach_id = media_handle_upload( $file_handler, $post_id );


//making it featured!
set_post_thumbnail($post_ID, $attach_id );
or
update_post_meta($post_id,'_thumbnail_id',$attach_id);
?>