When a sub-task is closed, the labels of the sub-task will be added to the Labels field of the parent issue.
Select parent of current issue.
The parent of the current issue will be updated.
Select Labels.
In the popup field:
Set the Value* to Set field value manually (parser expression) and insert the following expression:
%{parent.labels}, %{issue.labels}
The current values of the Labels field will be added to the parent issue's Labels field.
Choose which user* will be able to execute the post function. By default, it is set to the Current user.