Horde Mail? Can't get into Webmail

[KienN]

Hi! I'm a newbie with this Plesk 9.2
My company has just purchased this product for our own Hosting service.

I've been trying Horde Mail many times but everytime I browse the Webmail, it displays errors.
I think I get errors because I haven't created a domain called webmail.mydomain.com yet!

Can I set the Webmail url another domain??? for ex: my domain is kienh.com, my webmail url is hkien.com

If so, how can I do it in the DNS settings?

Please help me! Thanks so much!