JSFiddle - React, Tailwind, and code Playground

HTML

<form name="payForm" method="post" action="https://www.siampay.com/eng/payment/payForm.jsp">
<input type="submit" name="submit" value="Buy">
<input type="hidden" id="amount" value="<%= amount %>">
<input type="hidden" name="merchantId" value="76117579">
<input type="hidden" name="orderRef" value="76117579">
<input type="hidden" name="currCode" value="764" >
<input type="hidden" name="successUrl" value="">
<input type="hidden" name="failUrl" value="">
<input type="hidden" name="cancelUrl" value="">
<input type="hidden" name="remark" value="">
<input type="hidden" name="lang" value="E">
</form>

<button id='cmdSend'>Click me</button> 
<input id='howmuch' type="number" name="amount" value=1000>

JavaScript

document.getElementById("cmdSend").addEventListener("click", myFunction);
function myFunction() {
    document.getElementById("cmdSend").innerHTML = "YOU CLICKED ME!";
    
    var Value = document.getElementById("howmuch").value;
    console.log(Value);
    
    document.getElementById("amount").value = Value;
    document.getElementById("amount").setAttribute('value',Value);
    console.log(document.getElementById("amount").value);
}