Update: I found out how to add macros to the DefaultValue property of a field from reading this: https://docs.kentico.com/display/K8/Reference+-+Field+editor#Reference-Fieldeditor-Settingdefaultformfieldvaluesthroughmacros.
I tried {%cmscontext.currentdocumentparent.nodeid|(identity)GlobalAdministrator%} and it too is null. This doesn't make sense because the page I'm creating is nested under a parent container.