DNS Forwarding iRule on F5 GTM Listener

Hello,

I currently have a GTM that answers for all the DNS queries. But what I want to do, is to be able to for particular zones, to be able to forward to external DNS server. Given that add those zones to Data group and match for those zones in the iRule, what would my iRule look like? I am specifically interested about the forwarding part

when DNS_REQUEST {
    set forwarded 0
    if {[class match [DNS::question name] ends_with forwarded_zones]}{

.....

.....

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How can I explicitly set forwarding to specific forwarding IP or list of IPs in the forwarders list?

I am using 11.5.8 Version BIG IP software.

Thanks in advance