Once the Restricted People Manager has accepted the invitation to Runn, you can see their name from the Person Details side panel.
From the People Planner, find the project and click on the 3-dot icon next to the Person Name
Select Edit Details
Find the form field called Managers (optional)
You can also add the Restricted Manager from here. From the dropdown, select the Manager or Managers that you would like to assign. Then click Save.
Only Managers with People Access set as Restricted can be selected from dropdown menu.
For Managers with People Access set as All, they will by default have the right to edit this person and cannot be selected from the dropdown.