Form input value based on option value selected

Answer 1

Try....

$('select[name=filter]').change(function() {
  var input_field = $('input#field');
  switch ($(this).val()) {
    case 'A': input_field.val('Value if A is chosen'); break;
    case 'B': input_field.val('Value if B is chosen'); break;
  }
});