The MQTT broker is not available as an HA add-on. You will have to install the Mosquito MQTT plugin for TrueNAS. You will also install the MQTT integration in HA.
The MQTT client settings on the LinkTap gateway will use the Home Assistant host type. The broker address will be the IP of the TrueNAS MQTT plugin. Username and password will be your credentials for the MQTT plugin or blank if you didn’t configure them. Everything else is to be configured as instructed.
The MQTT integration will use the information of the MQTT TrueNAS plugin.
Yeah, it was definitely helpful. It did take me a beat to figure out how to get it to work with my installation. I don’t think I ever would have figured it out without their document. They only mention Alexa and Google Assistant in their advertising. I was trying to integrate the valve using one of those when I found that document.
They have a number of instructions for automation at the very bottom of the product page. Not really an obvious place to find them.