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Hereby is used in connection with what is present with the speaker or what the speaker has just mentioned or the speaker's present words (i.e. the present document); thereby is used in connection with what is at a distance from the speaker or what has been mentioned by the speaker or by someone else.
You are correct to doubt the placement. As hereby is an adverb, we like to place it near the verb. I would hereby like to announce .... However, hereby and by this letter mean the same thing. I would use one or the other, not both. There are other problems with the sentence as well, but they take us far from the topic of this thread.