Webservice published over Virtual IP redirects to IP address for authentication

I try to publish a complex web application for demo over our Fortigate 240D.

I use virtualIP to forward all the incoming http and https requests to our interal nginx load balancer.

I secured the virtual IP with the policy on the attached image. The most important part of the policy is that it is published only for the members of our sslvpn_group.

When user coming to the virtual IP https://mydemo.mycompany.com, it is redirected to [link]https://195.231.181.18/fgtauth?01002d0ea171e22a[/link] for authentication.

The problem is that certificate doesn't match with the ip address, the browser shows NET::ERR_CERT_COMMON_NAME_INVALID.

Is it possible to redirect to specific domain name instead of ip address ?

Best Regards,

János