[id10t]

Something like this:

RewriteEngine on
RewriteCond    %{REQUEST_FILENAME}  -d
RewriteRule    ^(.+[^/])$           $1/  [R]
RewriteRule   ^/webmail https://your.ispconfig.dns.name:81/roundcubemail [R,L]

In the Apache Directives field of the site would do it