Script to (1) default the Country code to "US" if it's in the Salsa form, or (2) append a Country code field with a value of "US".

country_is_us.html

<script type="text/javascript">
  // See https://help.salsalabs.com/entries/22876904

  // Actor.  Changes the value of any empty Country codes to "US".
  // If there's not a country code, then one is appeneded to the Salsa
  // <form> tag.  The appended country code is also defaulted to "US".
  function actor() {
    var country = $('*[name=Country]');
    if (country.length == 0) {
        var form = $('.salsa form');
        if (form.length > 0) {
            form.append('<input type="hidden" name="Country" value="US"/>');
        }
    } else {
      if (country.val().length == 0) {
          country.val("US");
      }
    }
  }
  
  // If we're on the letter page, then invoke the actor.
  // This gets called on a timer.  The timer is cleared when
  // after the actor runs.
  var checkExist = null;
  function recipientChecker() {
    if ($('.recipient').length > 0) {
      actor();
      if (checkExist != null) {
        clearInterval(checkExist);
      }
    }
  }   

  $(document).ready(function() {
    if (RegExp('action_KEY=\\d+').test(window.location.href)) {
      // For blind targeted actions and petitions.
      actor();
      // For targeted actions.  We must wait for the letter page to
      // appear to invoke the actor.
      checkExist = setInterval(recipientChecker, 200);
    } else {
    // Non-action pages.
      actor();
    }
  });
</script>