Answer
Open the page in Page Designer, find the relevant container/group element in the left panel, expand its Content, then expand the specific field you want to make mandatory, and check its 'Required' box. Save the change, then Publish it. After refreshing Aurena, the field will now be enforced as mandatory, matching the IEE behavior — mandatory-field enforcement in Aurena is a per-field Page Designer setting, not something that's automatically inherited from the equivalent IEE window's own mandatory-field configuration.