What errors are you seeing ? Maybe because of "starts_with" but you are not using the underscore.
You can break it down like this or maybe use switch statements:
when HTTP_REQUEST {
if { [HTTP::host] eq "support.domain.org" } {
if { [HTTP::uri] starts_with "/customers" } {
HTTP::redirect "https://support.domain.org/servicedesk/customer/portals"
} elseif { [HTTP::uri] starts_with "/agents" } {
HTTP::redirect "https://support.domain.org/secure/Dashboard.jspa"
} else {
HTTP::respond 404 content "Unrecognized request to [HTTP::uri]" "Content-Type" "text/html"
}
}
}