function maybePrefixUrlField () {
  const value = this.value.trim()
  if (value !== '' && value.indexOf('http') !== 0) {
    this.value = 'http://' + value
  }
}

const urlFields = document.querySelectorAll('.mc4wp-form input[type="url"]')
for (let j = 0; j < urlFields.length; j++) {
  urlFields[j].addEventListener('blur', maybePrefixUrlField)
}