To create a Light Portal:

The light portal should be oriented so that it is pointing in through the window as indicated below:

Light portal pointing inwards outside window

Skydome lights also have a portal_mode parameter with options off, interior_only and interior_exterior to respectively turn off portals, block any light outside portals for interior only scenes, and let light outside portals through for mixed interior and exterior scenes. Blocking light outside portals more predictably reduces noise for interior only scenes.