how can I associate a activation key to a host group or ??

I am not really clear on how to attach a activation key to whatever it needs to be able to add subscription to a newly provisioned host.

right now I am able to provision a new host via PXE and TFTP and it will install Red Hat 7.1 but the new host will not be registered and will not have any subscriptions available to it .

I have looked thru all the instructions and I really am at a loss to figure out how to do this

it says on the activation key screen

This activation key may be used during system registration. For example:
subscription-manager register --org="sarod" --activationkey="SXE-PI-DEVEL-Activation-Key"

but how can I get it to automatically run this command and get it to subscribe???
thanks