Question

How can I pass input from a text-type input element into a hidden input element in the same form on a site that has jquery installed?

Answer 1

Triggered when the text changes:

$("#text_id").change(function(){
  $("#hidden_id").val($("#text_id").val())
});

Triggered when the form submits:

$("#form_id").submit(function(){
  $("#hidden_id").val($("#text_id").val())
});

Check out other jQuery Events.