how do i change webmail url to url:port

[blackmetal]

Hello,
i want change the plesk 9.5 virtual host from webmail.domain.com to webmail.domain.com:9001 or domain.com:9001 for example, how should i do this?
which virtualhost should i edit?
Thanks,

 

[blackmetal]

i try edit /etc/httpd/conf.d/horde_vhost.conf to my own port and restart apache but it does affect ,
how should i do this?