The below excluding the creation of the $order_id_inpro is all inside an PHP echo.
$order_id_inpro = "1234";
<a class="dropdown-item" onclick="myFunction2('.$order_id_inpro.')">Upgrade Order</a>
At the bottom before my </body> I have the below script.
<script>
function myFunction2() {
var msg2=encodeURIComponent(this.id);
var myWindow = window.open("https://myurl.com/OaJER1?orderNumber="+ msg2 , "MsgWindow", "width=650,height=1000");
}
</script>
The results is that I get "orderNumber=undefined" at the end of the URL.
The onclick is passing the order number as a function argument, you need to get it using a function parameter.
function myFunction2(msg2) {
var myWindow = window.open("https://myurl.com/OaJER1?orderNumber=" + msg2, "MsgWindow", "width=650,height=1000");
}Inside the function, this is the global window object, so this.id is window.id, which is undefined.