You can do it with anyone as long as you use their birth name. If they have a nickname, you will have to remember their birthname...

For example, for my current crown prince, Prince Lorenzo de la Reza, now named Prince Lorenzo the Crusader, I'll have to type give_trait "Lorenzo de_la_Reza" Factionheir. You'll have to have a space between his main names, and underlining between his prepositions(wrong word?) and name.