It's a trick question. :) If someone else had already decided the meaning of your life, you would prefer that, because they would have decided what you preferred, ultimately.
Could they could arrange it so that your purpose was X, and that you preferred that your purpose was not X? I'm not sure if that's actually consistently possible. It's certainly possible for humans to want X and not want to want X, but something with a specified highest goal might not actually be able to hold both.