You have to eliminate these wanted targets, preferably using a knife. If they're executed using takedowns, you'll earn more EXP. You can however, kill them with conventional means and complete the quest however you'll only get ample rewards.
You can approach their camp and kill the guards stealthily or just go loud, eliminate everyone and leave the target for last before taking him out with a takedown. Like Path of the Hunter quests, Wanted quests also become available in bulletin boards of the liberated outposts.