HOW TO ADRESS A PRINCE ?

When you meet for example His Highness Prince Waldemar zu Schaumburg-Lippe, you say if you want to say something directly to him or write a letter

YOUR HIGHNESS

or when you speak in the presence about the prince to a third person> HIS HIGHNESS

then you either can say Prince Schaumburg-Lippe but alos Prince Waldemar is possible to keep it shorter and more personal.