Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [Human Resources Rep],
I would like to inform you of a situation I recently experienced that I felt was harassment. Without naming names, I would like to provide you with the following information:
On [date of incident] I was in [location of incident] when I was approached by [another employee/a supervisor/a manager/a customer/a vendor]. This individual proceeded to involve me in a conversation which was very uncomfortable, and [he/she] engaged in activity which I felt was not at all professional.
I am not sure how I should handle this incident, and would be very grateful if you would let me know when a good time for me to come by your office would be. I would like to talk with you in private about this incident.