Go to the Tavern and make a save. Ask the suspicious man for a job. Keep reloading the save until you are given a job where the Suspicious man needs someone with sniping skills to assassinate a target. Do this 10 times to unlock the mission.
Take note that this only unlocks if the save file recognizes if the window assassin contract has been done 9 times beforehand. This means that after completing one assassination, reloading the file without saving that progress will lose that result, and would have to be done again.