add this on translation >> Custom Home Message [Homepage screen settings]
change whatsapp no to your no
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.5.0/css/font-awesome.min.css">
<a href="https://api.whatsapp.com/send?phone=+919735475972&text=Hello" class="float" target="_blank">
<i class="fa fa-whatsapp my-float"></i>
</a>
add this on custom CSS option
.float{
position:fixed;
width:40px;
height:40px;
bottom:80px;
right:20px;
background-color:#25d366;
color:#FFF;
border-radius:50px;
text-align:center;
font-size:20px;
box-shadow: 2px 2px 3px #999;
z-index:100;
}
.my-float{
margin-top:12px;
}
