Send WhatsApp msg in HTML , JS

<html>
<head>
<title>WhatsApp msg</title>
<script>
function send(){
x="https://api.whatsapp.com/send?phone=";
abc=x.concat(document.getElementById("a1").value,"&text=",document.getElementById("b1").value);
window.open(abc);}
</script>
</head>
<body>
<div>
<fieldset>
<legend>Message</legend>
<input type="text" placeholder="phone number" id="a1" value=""/><br/>
<textarea rows="10" cols="20" placeholder="Type your msg here..." id="b1" value=""></textarea><br/>
<button onclick="send()" value="Send">send</button>
</fieldset>
</div>
</body>
</html>