gotcha.css

.gotcha {
  width: 100%;
}

.gotcha:after {
  content: "OPT OUT OF OUR WEEKLY NEWSLETTER";
  padding-left: 40px;
}

.gotcha:checked:after {
  content: "I WANT TO RECEIVE OCCASIONAL EMAILS ABOUT NEW PRODUCTS, PROMOTIONS AND OTHER NEWS";
}

gotcha.js

document.addEventListener('DOMContentLoaded', function(){
  var form = document.querySelector(INSTALL_OPTIONS.form.selector);
  
  var checkbox = document.createElement('input');
  checkbox.setAttribute('type', 'checkbox');
  checkbox.setAttribute('class', 'gotcha');
  
  var submit = form.querySelector('input[type=submit]');
  if (submit){
    form.insertBefore(checkbox, submit);
  } else {
    form.appendChild(checkbox);
  }
})

install.json

{
  "resources": {
    "head": [
      {
        "type": "style",
        "src": "./gotcha.css"
      },
      {
        "type": "script",
        "src": "./gotcha.js"
      }
    ]
  },
  "options": {
     "properties": {
        "form": {
          "title": "Form",
          "type": "object",
          "format": "element"
        }
      }
    }
  }
}