How can I express my motivation to join a Masters program if I have no experience in the area?
I assume that the new area isn't just a random choice. Think about what it was about that area that drew you to it and say some things about that. It would also be good to work in a bit about how your background and training makes this a natural trajectory for you. If you have read papers in the new area and find them interesting and think you can contribute to the ideas there, say something about that. 

Even your statement here that you want to broaden your horizons is positive, but, alone, it is worth only a little.