smart proxy must registered in Foreman (Infrastructure → Smart Proxies lists it with the TFTP feature)
you have a subnet defined that has TFTP proxy selected
you create a managed host (e.g. through UI) and have a provisioning interfaces assigned to the subnet from step 3
the network interface is also marked as managed
Once you save the host, Foreman creates a REST call to Smart Proxy that create’s a configuration for the MAC of the provisioning interface. Note that you must define many other things in order to create the host (domain, OS etc).