How do you properly invoke AddVisibilityPermission within Thingworx REST API properly?

Hello! Currently, I'm playing around with the Thingworx REST API.

I am trying to connect a ThingShape's visibility to an organizational unit. The highest Unit is "TopOrg" and the one directly below is "Administrator". Below that is "Worker", so TopOrg > Administrator > Worker.

According to documentation I need a "principal" and "principalType"

I've left my principalType to be "OrganizationalUnit", but I can't seem to get a correct "principal". I've tried "Administrator", "TopOrg:Administrator" and "TopOrg/Administrator". In either form, I get an error similar to this.

Unable to Invoke Service AddVisibilityPermission on AdminShape : Invalid Entity Reference: OrganizationalUnit/TopOrg/Administrator

How are principals properly formated?