JSFiddle - React, Tailwind, and code Playground

HTML

<form name="myform" action="/" method="post" >  
                    <input type="hidden" id="form_locatie" name="locatie" />
                    <input type="hidden" id="form_bonregel" name="bonregel" />           
                </form>         
                <table>
                    <tr>
                        <td>Locatie:</td>
                        <td><input type="text" id="locatie" onkeyup="CheckLocation()" /></td>
                    </tr>
                    <tr>
                        <td>Bonregel:</td>
                        <td><input type="text" id="bonregel" onkeyup="keyup(event)" /></td>
                    </tr>
                    <tr>
                        <td>Bonlijst:</td>
                        <td><textarea id="bonregelbox" readonly></textarea></td>
                    </tr>
                    <tr>
                        <td></td>
                        <td><input type="button" value="Velden Legen" id="reset" onclick="ClearFields()" /></td>
                    </tr>
                </table>

JavaScript

function GiveFocus(id)  {
			    document.getElementById(id).focus();
            }

            function CheckLocation() {
                if (event.keyCode == 13)
                    GiveFocus("bonregel");
            }

            function ErrorMessage(Message) {
                alert(Message);
            }
            
            function SubmitContent(locatie, bonregel) {
                document.getElementById("form_locatie").value = locatie;
                document.getElementById("form_bonregel").value = bonregel;
                document.myform.submit();
            }
            
            function ClearFields() {
                document.getElementById("locatie").value = "";
                document.getElementById("bonregel").value = "";
                document.getElementById("bonregelbox").value
            }
            
            function OnLoad() {
                var innerSectionText = document.getElementById("errorSection").innerText;
                if (innerSectionText != "")
                    ErrorMessage(innerSectionText);
                
                var textboxValue = document.getElementById("locatie").value;
                if (textboxValue != "")
                    GiveFocus("bonregel");
                else
                    GiveFocus("locatie");
            }
            
            function AddToList() {
                var bonregel = document.getElementById("bonregel");
                var val = bonregel.value.toString();

                if (val != "") {
                    var box = document.getElementById("bonregelbox");
                    if (box.value != "")
                        box.value = val + "\n" + box.value;
                    else
                        box.value = val + box.value;
                }
                bonregel.value = "";
                bonregel.focus();
            }

            var delayred = [];
            function delay(callback, id, calldelay) {
               ...