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I assume you have hosts registered in dnsmasq and you want to reference those using a local dns name.  In this case, you would set up query forwarding in Unbound to point queries with this domain to dnsmasq. This documentation contains an example: (https://docs.opnsense.org/manual/dnsmasq.html#dhcpv4-with-dns-registration).  I've been on dnsmasq for a while now and don't remember how this was done with ISC, but it might have been automatic before.

If my assumption is false and you are trying to do something else, then I don't know.

I use pihole as my primary dns servers for clients with unbound on OPNsense set as the upstream dns server on pihole.  The advantage of this setup is that you get local host resolution since OPNsense is able to get that from the DHCP server (ISC or dnsmasq).

EDIT:  After reading more carefully, I see that you can't reach the pihole directly.  If the default gateway is OPNsense, then maybe you can port forward those DNS requests to pihole.  I am also doing that to force all DNS to pihole.

Unbound on port 53, DNSmasq on port 53053, and set up the Unbound query forwarding in accordance with OPNSense docs:
https://docs.opnsense.org/manual/dnsmasq.html

I followed the examples at that link for my configuration, and it's running flawlessly for me across 5 different interfaces. Unlike the first person who responded to you, I feel like this was a pretty rock solid initial release for a lighter and more efficient DHCP. From what I've gathered between here and Reddit, the majority of the people having issues decided to wing it with their setup and didn't read the guides first.

I followed this guide and it mostly works, however I am having a problem with nslookup for the host overrides (reservations).  I get unable to communicate with DNS error messages when doing nslookup on the hosts that have reserved addresses.  nslookup works fine for the dynamically assigned hosts.  My setup is slightly different in that I am using two pihole DNS servers with unbound on OPNsense as the upstream DNS server.