Sorry, I send a screenshot from the MQTT Transmission module. It's the Primary Host ID in the screenshot you need to set.
You don't need the primary host buffer setup to store forward, it's just an extra "feature" to make sure that you store forward if node-red is connected to the MQTT broker, but the primary application (in this case MQTT Engine) is not. You can just choose to only store forward when node-red is not connected to the broker.
If you want a better understanding of the sparkplug protocol and how the primary application works, then I would recommend that you read the sparkplug specification:
You can send is_historical attribute with all the nodes (but I'm pretty sure you don't want to). It's not really documented anywhere but the attribute needs to be named isHistorical.