Custom script only works when website is refreshed not on first load.

Site URL: http://www.hausline.be/contact

I have a problem where the script that changes the default values of the contact form (first name and last name) are changed to my language, but it is only executed when I refresh the page. On first load the scripts doesn't change anything. 

This is the code I used, after some extensive searching here on the forums.

<script>
function addLoadEvent(func) {
 var oldonload = window.onload;
 if (typeof window.onload != 'function') {
   window.onload = func;
 } else {
   window.onload = function() {
     if (oldonload) {
       oldonload();
     }
     func();
   }
 }
} 
addLoadEvent(function() {  
 sqsFirstName = document.getElementsByClassName("caption")[0].innerHTML;
 myFirstName = sqsFirstName.replace("First Name", "Voornaam");
 document.getElementsByClassName("caption")[0].innerHTML = myFirstName;

 sqsLastName = document.getElementsByClassName("caption")[1].innerHTML;
 myLastName = sqsLastName.replace("Last Name", "Naam");
 document.getElementsByClassName("caption")[1].innerHTML = myLastName;

});
</script>

Anyone know where I went wrong? Thanks.

Try disable this: https://support.squarespace.com/hc/en-us/articles/115000253288-Ajax-loading#toc-enable-or-disable-ajax

Thank you, that did the trick I think :-).