JSFiddle - React, Tailwind, and code Playground

HTML

<tr>
    <td>
        Facebook:
    </td>
    <td>
        <input style="width:300px" type="text" class="facebookUrl" name="facebook" value="http://www.facebook.com/$facebook">
    </td>
</tr>

JavaScript

$('input.facebookUrl').keyup(function(){
        if (
            ($(this).val().length > 0) && ($(this).val().substr(0,24) != 'http://www.facebook.com/')
            || ($(this).val() == '')
            ){
            $(this).val('http://www.facebook.com/');    
        }
    });