Edit in JSFiddle

<form>
  <div>
    <p>Select country</p>
    <input type="radio" name="country" value="NG" no-js on-click-set-value="[name=number] +234"> Nigeria
    <input type="radio" name="country" value="GH" no-js on-click-set-value="[name=number] +233"> Ghana
  </div>
  <br>
  <br>
  <div>
    Phone Number
    <br>
    <input type="text" name="number" placeholder="Phone number">
  </div>
</form>

body {
  font-family: sans-serif;
  background: white;
  color: black;
}

input[type=text] {
  margin-top: 20px;
  padding: 10px;
  min-width: 200px;
}

// no javascript.