htaccess to redirect www.sudomain.domain.com to

[internetbrother]

I want my users to be able to see their site when the type

www.subdomain.domain.com

but now it does not work and only it works when they type
subdomain.domain.com

[Nicolas]

Maybe add a subdomain in the subdomain.

Then put an index.php file in there, or html that sends them to the right one.

Not sure about htaccess though. Never touched it in my life

[pentapenguin]

Depending on the way your host has the server setup, the www part may point to a sub-subdomain rather than the actual subdomain. There may be an option in your hosting control panel to forward the www and no www to the same address.