Fortigate webauth authenticated policies

Hello,

I am new with Fortigate product, i have a 900D firewall and i want to migrate from Juniper SSG screenOS where we had multiple policies which was configured for authentication and we had a webauth portal ip where the authentication was made.

For example before accessing some resources that had webauth policy i was entering in a browser something like https://webauth and prompts for authentication. After that a successful message appear and the resources were available.

On fortigate on the other hand i didnt found that option beside some Captive Portal that i can use but i cannot control when i want to authenticate, it pops up when a HTTP, HTTPS or a FTP connection is detected but i need the webauth for several other ports or protocols and i cant find a solution for that.

Is someone familiar with both screenos and FortiOS who can help me with this?

Thanks!