I've figured out that openpanel does not provide that posibility.
So I had to do it manually.
in the following folder:
/etc/apache2/openpanel.d
your domain (or subdomain -> we will take sub.domain.com as example) will have a specific config file:
sub.domain.com.conf
open it
<VirtualHost *:80>
DocumentRoot /home/user/sites/sub.domain.com/public_html/
ServerName sub.domain.com
<Directory /home/user/sites/sub.domain.com/public_html/>
AllowOverride All
Allow from all
</Directory>
Include /etc/apache2/openpanel.d//sub.domain.com.inc/[^.#]*
Include /etc/apache2/openpanel.d//global.inc
</VirtualHost>
change it to:
<VirtualHost *:80>
DocumentRoot /home/user/sites/sub.domain.com/public_html/web/
ServerName sub.domain.com
<Directory /home/user/sites/sub.domain.com/public_html/web>
AllowOverride All
Allow from all
</Directory>
Include /etc/apache2/openpanel.d//sub.domain.com.inc/[^.#]*
Include /etc/apache2/openpanel.d//global.inc
</VirtualHost>
Note that in the line I've added "/web/" in the line 2 and "/web" in the line 4