Virtuemart character counter for meta boxes (on admin page; meta title, meta description etc.)

Question

How can I put a character counter in the meta boxes (meta title, meta description) on the Virtuemart admin product adding page?

(Only visible on admin page)

or

Can I get the file path of the product editing page back-end the site?

Best regards

For example, I couldn't find where to add this:

$simple = "$meta-title-box";

echo strlen($simple);