JSFiddle - React, Tailwind, and code Playground

HTML

<input type='text' id='ordernumber' onchange="javascript:takeawayfield()" onkeyup="javascript:takeawayfield()" />
<select id='system' onchange="javascript:takeawayfield()">
    <option>sys1</option>
    <option>sys2</option>
    <option>sys3</option>
</select>

<div id='Memo' style='display:inline'>Memo</div>

JavaScript

var dok = 0,
    initialValue = "",
    ordernumber = null,
    system = null,
    memo = null;

window.onload = function() {
    // get dom objects
    ordernumber = document.getElementById("ordernumber");
    system = document.getElementById("system");
    memo = document.getElementById("Memo");
    
    // set initial value
    initialValue = ordernumber.value;
};

function takeawayfield() {
    if (ordernumber.value != initialValue && system.value == "sys1") {
        dok = 0;
        memo.style.display = "none";
    } else {
        dok = 1;
        memo.style.display = "inline";
    }
};