You create a Machine Template, and assign a runtime to that template. The connect UiPath Assistant using the details from that machine template. Separately, create a Robot account with credentials to login to the unattended robot PC.
Assign both the machine template and robot account to the modern folder where the processes are located. Then run a process on the PC and see how it goes.
Once UiPath Assistant is connected it says Unlicensed because the “Unlicensed” is referring to the ability to run processes in an Attended way (i.e. somebody manually clicking a process to run on UiPath Assstant). You should still be able to trigger processes to run from Orchestrator to that PC.