Conditional dropdown in HTML form to limit states by selected country

In an HTML form, create a dropdown list conditional on the choice in another dropdown list. In our form we ask for the user's country, city, and state. We need the choice of state to be limited by the choice of country without refreshing the page. So if you pick United States, you get all valid US states. If you pick a country that doesn't have states, then the state options disappear (and submit a blank entry for that field in the form). The website is in SquareSpace with a custom HTML form at [login to view URL] You must also preserve all the submit, post, and Captcha features already in place.