Tampermonkey script - Refresh needed

Question

I'm trying to create a simple script to autoclick a buy button as soon as the browser goes from the cart page to the checkout page. The script doesn't work unless I manually refresh the checkout page. It appears the site uses AJAX.

I started with this simple script

// ==UserScript==
// @name         Auto place order
// @version      0.1
// @description  Autoclicks on buy button
// @author       You
// @match     https://www.mywebsite.com/buy/placeOrder
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
alert("Matched"); //debug
const buybutton = document.querySelector('#submitOrderButtonId-announce');
if(buybutton) {
  buybutton.click()
}
})(); 

Upon reading other suggestions I tried the below

// ==UserScript==
// @name         Auto place order
// @version      0.2
// @description  Autoclicks on buy button
// @author       You
// @match        https://www.mywebsite.com/buy/placeOrder
// @grant        none
// @require     http://ajax.googleapis.com/ajax/libs/jquery/2.1.0/jquery.min.js
// @require     https://gist.github.com/raw/2625891/waitForKeyElements.js
// ==/UserScript==

waitForKeyElements ("#submitOrderButtonId", () => {
alert("Matched");
document.querySelector('#submitOrderButtonId-announce').click(); });

but it didn't help. Still needs refresh.

If I use the main domain in @match it works but I can't set it like that.

Could anyone please suggest the exact code I should write to avoid refreshing?