How to Sideload Featured Image on Frontend Submission?

I have a custom post type of "Websites" and a Front-end Submission form to add a website. I use the following that works via the WP Admin Panel. How can i trigger the same type of functionality on my website frontend submission form?

function download_featured_image( $post_id ) {

if ( has_post_thumbnail() ) {
    return;
}

// example image
$image = "https://cdn.restpack.io/a/cache/st/screenshot/6dba6142c1b6e0930f0b201f4dc3fb37c01d2f73";

// magic sideload image returns an HTML image, not an ID
$media = media_sideload_image($image, $post_id);

// therefore we must find it so we can set it as featured ID
if(!empty($media) && !is_wp_error($media)){
    $args = array(
        'post_type' => 'attachment',
        'posts_per_page' => -1,
        'post_status' => 'any',
        'post_parent' => $post_id
    );

    // reference new image to set as featured
    $attachments = get_posts($args);

    if(isset($attachments) && is_array($attachments)){
        foreach($attachments as $attachment){
            // grab source of full size images (so no 300x150 nonsense in path)
            $image = wp_get_attachment_image_src($attachment->ID, 'full');
            // determine if in the $media image we created, the string of the URL exists
            if(strpos($media, $image[0]) !== false){
                // if so, we found our image. set it as thumbnail
                set_post_thumbnail($post_id, $attachment->ID);
                // only want one image
                break;
            }
        }
    }
}

}
add_action( 'save_post', 'download_featured_image');