JSFiddle - React, Tailwind, and code Playground

HTML

<select name="phoneCallingCode" id="phoneCallingCode" class="input">
   <option value="93">Afghanistan (+ 93)</option>
   <option value="355">Albania (+ 355)</option>
   <option value="213">Algeria (+ 213)</option>
</select>
<input type="text" id="ShowNumber"/>

JavaScript

$('#phoneCallingCode').change(function() {
    $("#ShowNumber").val("+" + $("#phoneCallingCode option:selected").val())
});