I'm assuming the previous correspondence went to your home?(posted 8266 days ago)A. IF so, then just this single letter counts as harassment; they know where you live, and have no right to harass you at work. end of story.
or B. IF you've been corresponding so far from at work, then its not harassment (IMHO), they are entitled to ask, provided they are polite, for you to complete a form. Equally, you are equally entitled, politely, to say no. They've got no legal right to the information they are asking for, you've got EVERY legal right for the information you are asking for.
If you fall into category B my advice, for what its worth, is to write back declining and refer them to your earlier letter, to which you are still awaiting a response.