After authenticate on captive portal, show an warning mensage for insecure network

Hello,

I performed captive portal authentication on external server fortiauthenticator, and whant use a open wifi and restrict access for captive portal. config is ok, i think, but, after the login, before redirect to desirable URL, web browser show an warning mensage of insecure conection, and ask to accept the risk. shows URL http://<ip interface fortigate>:1000/fgtauth. this page present an fortiAP certificate. How to correct this for that warning mesage dont show?