We have a public access SSID that needs to be setup so all the users are filtered. How do we configure the SSID or Role so that when they access the internet they go though our proxy server for filtering.
what you need is a transparent proxy. Create a separate VLAN for these users and place your proxy server in-line. You can also configure your DHCP to provide the proxy server address as default gateway on that VLAN.
You'll create a new Role that places Users from the public SSID into that VLAN.
In that scenario, all traffic from the clients will be routed through your proxy server. But you need a transparent proxy for that. If you only have a Web-Proxy for http(s) traffic than this setup might not work as expected.