As I mention in my article on requirements, it is important to specify the "what" instead of the "how" when documenting product requirements. Specify the criteria you would use to evaluate whether the product is likely to succeed in the market, but give as much latitude as possible to designers and implementors to develop a product that satisfies the criteria.
Looking to the more general issue of problem-solving helps us understand why this advice is so important. Most problem-solving books emphasize the importance of a clear definition of the problem and creative brainstorming. The idea is that the best and most innovative solutions to problems often result from being open minded. When you are unable to think of a problem independently of solutions to it, you hinder your ability to thoroughly understand the problem and creatively address it.
That said, exploring solutions to a problem can actually help us gain a more thorough understanding of the problem and its effects. So it is not necessarily wrong to explore solutions and perhaps even in rare cases suggest or dictate them as product manager. However, at some point along the way, you should still have defined the requirements in a manner unbiased by preconceived solutions.