OPNsense, and FreeBSD more generally, are community supported and built from the Tailscale client code (which is open source). However some of us do run OPNsense ourselves, including me.
The instructions set up tailscale for just the OPNsense host, so you can (for example) remotely connect to its web UI. If you’re expecting OPNsense to route from your LAN to the tailscale network, that can be done but requires an additional option when bringing the network up: https://tailscale.com/kb/1019/subnets
You’ll need to accept the subnet routes in https://login.tailscale.com/admin/machines before they will work. If everything is configured correctly you’ll see a “Subnets” badge on the machine. A screenshot from my admin panel is attached.
One note, however: Subnet Routing isn’t a feature in the free Solo plan. https://tailscale.com/pricing/
(I pay for the Connectivity plan for my personal Tailscale use.)