JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" placeholder="Your personal message" id="valbox"></input>
<input type="button" name="design1" id="butval" value="Choose Design"></input>
<input type="text" class="input-text" name="billing_last_name" id="billing_last_name" placeholder="" value="">

JavaScript

window.onload = function(){
    document.getElementById('butval').onclick = function(){
        document.getElementById('billing_last_name').value = document.getElementById('valbox').value;   
    }
};