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="nmbr">

CSS

ul{ list-style: none; }
a{ text-decoration: none; }
#menu{
	margin: 60px 0 0;
	padding: 0 0 10px;
	z-index:100;
	position:relative;
	background:white;
}
#menu ul li{
	padding: 2px 0;
	position:relative;
}
#menu ul li a{
	color: #333;
	font-size: 18px;
}
#menu ul li a:hover,
#menu ul li.current_page_item a{
	color: #10b0e6;
}
#menu ul li ul{
	display:none;
}
#menu ul li.menu-item-has-children:before{
	content: "";
	height:0;
	width:0;
	position:absolute;
	border: 3px solid transparent;
	border-top-color: #999;
	top: 14px;
	left:-20px;
	z-index:1;
}
#menu ul ul{
	margin: 2px 0 5px 10px;
}
#menu ul ul li{
	padding:0;
}
#menu ul ul li:before{
	top: 10px !important;
}
#menu ul ul li a{
	font-size: 15px;
	color: #666;
}
#menu ul ul li a:hover,
#menu ul ul li.current-menu-item a{
	color: #49B869;
}
#menu ul ul ul li a:hover,
#menu ul ul ul li.current-menu-item a{
	color: #F9A64A;
}
#menu ul ul ul li a{
	font-size: 14px;
}

JavaScript

//For starting of page
  valNum=$("#phoneCallingCode").val();
   $("#nmbr").val(valNum);

$("#phoneCallingCode").change(function(){
   valNum=$(this).val();
   $("#nmbr").val(valNum);
});