If I understand the "Message in a box" technique correctly, all that is required is for the desire to be launched (i.e. putting the message into the box), and then forgetting about it. So theoretically, no belief is required, since part of the technique is forgetting about it until you notice it show up in your experience.

The larger philosophical problem is that the Law of Attraction is essentially untestable since, if the manifestation does not occur, the devout will tell you that you simply lacked the necessary desire and belief, and that is why your manifestation didn't occur.

In other words, you're testing your faith, not the process itself.

This is less mystical than it sounds. People who practice any discipline have knowledge about that discipline that emerges from long practice with the tools of their trade. Whether you are a tennis player or a concert pianist, you started out with no knowledge at all. In the beginning, you had to have some faith that you were even capable of accomplishing it, but over time, that skill has become second nature to you.

So it is with manifestation. Those who practice it regularly, and understand how it works on an intuitive level, are able to perform better (i.e. manifest things consistently) than those who don't.

Some people will argue that the discipline, training and practice are not required, that you can simply "channel" the knowledge of a concert pianist. That's fine, but most people would like to call the skill their own, and be able to call on that skill at will in this physical platform we call life. In other words, they want it to be "real" and not "imaginary." And concert pianists and tennis players are doing more than just putting a message into a box.

Which is not to say that imagination, desire and belief are not important ingredients; they are. In fact, they are essential; you can't do anything without these ingredients. You must first believe you can do it, and have the desire to accomplish it, before you can manifest it.