JSFiddle - React, Tailwind, and code Playground

HTML

<select name="country" id="country">
    <option value="">Country...</option>
    <option value="966">Saudi Arabia</option>
</select>
<br /><br />
<label for="phone">Phone Number</label>
<input id="phone" name="phone" placeholder="user's number " required="" type="number">

JavaScript

$('#country').change(function () {
    var countryCode = $(this).val();

    if (countryCode) {
        $('#phone').val(countryCode);
    }
});