createForm.js

var formElement = document.createElement('form');
formElement.setAttribute('method','post');
formElement.setAttribute('action','login.php');

var usernameField = document.createElement('input');
usernameField.setAttribute('type','text');
usernameField.setAttribute('name','username');

var passwordField = document.createElement('input');
passwordField.setAttribute('type','password');
passwordField.setAttribute('name','password');

var submitField = document.createElement('input'); 
submitField.setAttribute('type','submit');
submitField.setAttribute('value','Submit');

formElement.appendChild(usernameField);
formElement.appendChild(passwordField);
formElement.appendChild(submitField);

document.body.insertBefore(formElement, document.body.childNodes[0]);