How do I change webmail.domainname.com to something else?

[Peach@]

This is what I'm trying to accomplish:

I have a SSL certificate for www.xxxxx.com. I want to be able to read Horde email using the SSL certificate for www.xxxxx.com. If I simply go to https://webmail.xxxxx.com, I get a warning. So I want to use https://www.xxxxx.com/webmail to be the URL to access Horde. Is it possible?

I tried inserting these lines in vhost.conf
ProxyPass /webmail/ http://webmail.xxxxx.com/
ProxyPassReverse /webmail/ http://webmail.xxxxx.com/

It works fine for the login page. When I login, though, the site switches back to http://webmail.xxxxx.com. Could it simply be a PHP code problem in Horde? I f so, where can I find the Horde pages on the server?

This is for one domain only. I don't want to do it for other domains on the same IP or server.
Thanks!

 

[Peach@]

OK, I found Horde files here:
/usr/share/psa-horde/

It looks like many pages in Horde call up $_SERVER['REMOTE_ADDR'] or $_SERVER['SERVER_NAME']. With ProxyPass an ProxyPassReverse, these variables return webmail.xxxxx.com. That may be why it's going back to webmail.xxxxx.com.

I also tried this, but this doesn't seem to work in vhosts.conf:
Alias /webmail/ /usr/share/psa-horde/

I get an error when I restar apache.

What else should I try?
Thanks!