iRule 404 redirect with original URL in URL bar

Hi. Sorry for asking but I cannot find a complete answer for this.

If a user browses to an invalid URL (e.g. https://www.example.com/invalidurl), I would like the iRule to redirect to a separate 404 webpage (e.g. https://www.example.com/404.html) but contain the original URL in the URL bar - i.e. the redirect to https://www.example.com/404.html is not visible to the user.

There are examples that redirect ok but the 404 URL is still visible to the user. Any advice please?