You do not need to brand a Captain beforehand. Simply enter any easy target's recruitment mission. Brand him before the first set of Uruks die. You can be seen by the enemy as long as you kill all the targets indicated by the yellow dot markers afterwards. After you brand the Captain, all the Uruks following him will be under his control, as if you had branded them yourself. Ensure that the Captain does not die from other groups of Uruks to allow success of the recruitment mission. When the mission ends, use the ability to kill all branded units on sight.