The HTTP IN node does not spawn a new express instance, it will use the same instance that Node RED is using, but with a different route (as configured in the configuration of the node).
So using a domain or not - you will need to specify a port on client devices.
That's not to say you can't use some form of tunnel/port forward to hide the true port - but you will need to research your options.
And remember to give this a read. 
Safely accessing Node-RED over the Internet - FAQs - Node-RED Forum (nodered.org)
