How to enable ssl vpn web mode access on the internal port

Hello all, 

when enabling ssl-vpn settings on the wan interface, at ssl-vpn settings automatically appears "web mode access will be listening at "https://ip:port", but when I choose the intenal lan interface for the same thing (I want people on the office to connect to servers with firewall authentication) it doesn´t show that option. 

How can I enable web mode access on the internal interface?