Under Policy > Policy Elements > Results > Authorization Profiles you should have whichever profile does your redirection. In it, under Common Tasks, you should have Web Redirection checked. Underneath that entry you should have another checkbox called "Static IP/Host Name /FQDN". If you check that, you get an input box that allows you to specify the FQDN. You can see the URL in Attributes Details to make sure it's what you want.
