Get place parent external_id in xlsform without using current user contact parent?

Consider the creation of the health_center place whose parent is district_hopital, how to retrieve the external_id of the district_hopital in health_center xlsform without using current user contact parent?
It is easy to get parent _id and name, but I can’t get other information about place parent. I need your help if it is possible.

type name label::en appearance calculation read_only default
begin group health_center Health Center field-list
string parent NO_LABEL hidden . PARENT
string parent_external_id District Hopital External Id ??? yes
string name Health Center Name
string external_id Health Center External Id
calculate code Health Center Code concat(${parent_external_id},‘-’,${external_id})
end group