The start requires that you talk with Mike in the bottom left corner of Kagabangui while Brad is in your party.

After the first events with Mike, you need the dialog about slavery with King Lancafew, to have fixed the border towers so that Dorgania opens their borders, and Sandra in your party. Once you have all of this, you can find a burning note in Kagabangui below some palm trees near the teleport stone plate in the center of the city. That note gives you the "Dorgania Slaves" quest.