Sorry for the very late response. I was out of state for many months and was unable to do my day to day task.
As for your question, the first link in the article provides steps to creating a global webmail page. Global Webmail url
You only need to tweak it to fit your needs.
You can use the following as a means to add the Horde dir for webmail:
Create a Symlink to the path you have Horde installed:
ln -s /var/www/web/site1/horde /var/www/webmail
Next edit the following file:
And add to the bottom of it:
Alias /webmail /var/www/webmail/
Options Indexes FollowSymlinks
Allow from all
This will now allow all of your clients to be able to hit the webmail from any URL pointed to your box.
Again, I know this response was late and the plan is to be more involved as ISPConfig has to be the absolute best Open Source Solution to hosting. I am excited to see this project 5, 10 years from now.