Using the new URL you've mentioned in the comments (please also update your post), you can do this with JavaScript and with a redirect through your own server.
Depending on your server-side scripting language, you may have to handle the server part differently. The following example is for PHP:
Client HTML:
<form method="GET" action="dhl_redirect.php" id="myForm">
<input type="hidden" name="id" value="" id='myId' placeholder="Enter tracking number" />
</form>
Client JavaScript:
(function(){
var form=myForm;
form.addEventListener('submit',function(evt){
var dhlUrl="https://dhlforyou.nl/login/track/";
var id=document.getElementById('myId');
if(/^\d+$/.test(id.value)){
if(myForm.target==='_blank'){
window.open(dhlUrl+id.value);
}else{
location.href=dhlUrl+id.value;
};
evt.preventDefault();
},false);
})();
Server:
<?php
if(isset($_GET['id'])&&!is_nan(intval($_GET['id']))){
$dhlId=$_GET['id'];
header('Location: https://dhlforyou.nl/login/track/'.dhlId);//Redirect to DHL
exit;//Skip the rest of the script
}else{//ID not set or not a number
header('Location: myHTMLform.htm');
}
?>
If you want to have this form open in a new tab, just set the attribute target
to _blank
.