Prophecy according to the Holy Spirit. Most prophecy is given when the watchman or prophet is not expecting it. Other times, the man himself prays for an answer to a question, and receives a prophetic word, or wisdom worth knowing. Even in these cases, it is usually the Holy Spirit who has first called to the man to ask in the first place.
To ask another man about where to prophecy... it is an unwise undertaking, in my opinion.
It has never even entered into my mind to do such a thing.
Seek and obey God. Walk in the Spirit.