First site I managed to use my debugger to add Mx was actually HMRC's website, back before they provided Mx as an option. I was delighted to receive confirmation of my tax return addressed to me by that title! Just keep trying, every time you find a terrible site. The one that annoys me the most right now is Confused.com, which requires that you select from "Mr", "Miss", "Ms", "Mrs", "Dr (Male)", or "Dr (Female)". I can see why they do so (some car insurers still make decisions based on gender, and some of those derive gender from title rather than asking for what they actually want to know), not least because I had a conversation with somebody from Confused.com about this particular policy (and they pointed a finger at insurers!), but it still gave me significant feelings of rage.