How to use fu_after_upload

Dear Community,
Where can I program things I want to do with the attachement IDs and the post ID after it gets submitted?

I want to use this code but I am confused where exactly I can use it:

add_action( 'fu_after_upload', 'my_fu_after_upload', 10, 3 );

function my_fu_after_upload( $attachment_ids, $success, $post_id ) {
    // do something with freshly uploaded files
    // This happens on POST request, so $_POST will also be available for you
}

https://wordpress.org/plugins/frontend-uploader/