To answer this question, let's start with the verb harass. According to three leading English dictionaries, harass can be pronounced with the stress on either the first or second syllable. But two of the dictionaries indicate that the pronunciation of harassment is with stress on the second syllable (the third does not indicate its pronunciation). This suggests that the pronunciation of harassment with stress on the first syllable is a carry-over from the variant pronunciation "HAR-ass" and may not be as well established as "ha-RASS-ment." Indeed, the pronouncing dictionaries I also consulted say to pronounce this word as "ha-RASS-ment." So I'd also advise pronouncing it this way, with stress on the second syllable.