require_custom_field.js

// EXAMPLE of ID in the URL https://subdomain.chargify.com/setup/metafields/1033/edit

$(document).ready(function(){
  var customLabel = $("label[for='subscription_metafields_XXX']");
  customLabel.text(customLabel.text() + " *");
});

var customInput = $("#subscription_metafields_XXX");
var form = $("#signup-form");
var submitBtn = $("#subscription_submit");

submitBtn.click(function(){
  if (customInput.val() === "") {
    customInput.addClass("field-error");
    return false;
  }
});

form.change(function() {
   if (customInput.val() != "") {
       customInput.removeClass("field-error");
       submitBtn.click(function() {
        return true;
       });
   } else {
      return false;
   }
});

.field-error {
  border-color: #ff0000;
}