create-tf-button.js

window.onload = function() {
  var loginButton = document.createElement('a')
  var loginButtonImg = document.createElement('img')

  var dataset = document.querySelector('script[name=tf-login]').dataset

  console.log(dataset['redirect_uri'])

  var oauthUrl = 
    'https://api.typeform.com/oauth/authorize?client_id='
      + dataset['client_id']
      + '&scope='
      + dataset.scope
      + '&redirect_uri='
      + dataset['redirect_uri']
      + '&state='
      + dataset.state

  loginButton.href = oauthUrl
  loginButton.setAttribute('target', '_blank')

  loginButtonImg.src = 'https://dummyimage.com/100x100/000/fff'
  loginButton.append(loginButtonImg)
  document.getElementById('tf-login').append(loginButton)
}