Example dependent dropdown code for support center / widget contact form.

gistfile1.html

<style>
.hidden {
  display: none;
}
</style>

<div class="div-category">
  {{ ticket_custom_category }}
</div>

<div class="div-subcategory hidden">
  {{ ticket_custom_subcategory }}
</div>

<script>
$(function() {
  $(".div-category select").on("change", function() {
    if ($(this).val() == "Alpha") {
      $(".div-subcategory select").show();
    } else {
      $(".div-subcategory select").hide();
    }
  });
});
</script>