JSFiddle - React, Tailwind, and code Playground
HTML
<input type="text" />
<p>facebook/<span>your.username</span> </p>
CSS
input{
width: 300px;
height: 30px;
}
span {
font-weight: bold;
}
JavaScript
$(function() {
$('input').on('keyup', function() {
var username = $(this).val();
if(username.length > 0) {
$('p > span').html(username);
} else{
$('p > span').html('your.username');
}
});
});