Issue Unable to setup RoundCube

[Pedro Cavaleiro]

Hi, I'm trying to setup RoundCube on my server but for now all I was able to do is to get it running on http://example.com/roundcube
I've tried to preform these steps

External Webmail Sevices
To register an external webmail service:

1. Go to Tools & Settings > Webmail (in the Mail group).
2. Click Register Webmail.
3. Specify the following:
   1. Webmail service name.
   2. Webmail service URL. Specify an address that begins with the http:// or https:// prefix.
   3. Leave the Available checkbox selected to make the registered webmail service available for selection in hosting plan settings.
4. Click OK.

Setting the url to example.com/roundcube
But when I try to access webmail.example.com it shows the default Plesk page as I'm a reseller for a second domain I have the exact same problem, I go to webmail.example2.net and I get the same default Plesk Page

Any ideas on how can I solve this problem?
NOTE: I have full access to my server (desktop access)