Here is the Country Code Select List JS, Please put this to the "Before Body End Tag"

country_code.js

<script src='https://cdnjs.cloudflare.com/ajax/libs/intl-tel-input/17.0.8/js/intlTelInput.min.js'></script>
<script>
// International telephone format
// $("#phone").intlTelInput();
// get the country data from the plugin
var countryData = window.intlTelInputGlobals.getCountryData(),
  input = document.querySelector("#phone"),
  addressDropdown = document.querySelector("#country");

// init plugin
var iti = window.intlTelInput(input, {
  hiddenInput: "full_phone",
  utilsScript: "https://cdnjs.cloudflare.com/ajax/libs/intl-tel-input/17.0.8/js/utils.js" // just for formatting/placeholders etc
});

// populate the country dropdown
for (var i = 0; i < countryData.length; i++) {
  var country = countryData[i];
  var optionNode = document.createElement("option");
  optionNode.value = country.iso2;
  var textNode = document.createTextNode(country.name);
  optionNode.appendChild(textNode);
  addressDropdown.appendChild(optionNode);
}
// set it's initial value
addressDropdown.value = iti.getSelectedCountryData().iso2;

// listen to the telephone input for changes
input.addEventListener('countrychange', function(e) {
  addressDropdown.value = iti.getSelectedCountryData().iso2;
});

// listen to the address dropdown for changes
addressDropdown.addEventListener('change', function() {
  iti.setCountry(this.value);
});
</script>
<script>
//Append Value To Phone Field
$("#phone").prop('value', '+1 ');
</script>