When you leave the Whoreson Junior's hideout in Oxenfurt after completing the "Get Junior" quest, a few Redaian soldiers will approach you and inform you that King Radovid wishes to meet you. They will then take you to the port where his ship is moored.
As you are, no doubt, already well aware, Geralt has always paid particular interest to crowns and the men who wear them. Kings, you see, are always in need of specialists to perform dirty jobs for them. When Geralt heard King Radovid wanted to see him, he surmised this was just such a moment and set off in the hope a profitable witcher contract was in the offing.
