JSFiddle - React, Tailwind, and code Playground

HTML

<div class="formsection clearfix">
<label for="title">Name</label>
<span class="directions">The name you want to be displayed in your listing.</span>
<input type="text" name="title" id="earnedSum" value="" size="50" maxlength="100">
</div>

<div class="formsection clearfix">
<label for="url_title">URL Title</label>
<span class="directions">Put a dash between first and last name. example: "john-smith"</span>
<input type="text" name="url_title" id="milestone-amount-1" value="" maxlength="75" size="50">
</div>

JavaScript

$(function(){
    $("#earnedSum").keyup(function(){
        $('#milestone-amount-1').val($('#earnedSum').text());

    });
});