Iframe content not showing - html

Question

I have an iframe without src. This because the iframe only shows webpage content (from the src) when a user clicks a button (the src is then replaced by a webpage URL). That button is already working.
But when nobody clicks the button, the iframe has to show some simple default text instead of that webpage. In my example I have written the text line 'This text must be displayed by default.'. Why is the text not showing up?
I need to keep the onload javascript, because that is important for the functionality of the iframe.
<iframe src="" class="prodframe pc" style='bottom:0px;position:absolute;width:90%;height:73%;margin:0px auto;left:0;right:0;border:none;background:none transparent;' frameborder='0' scrolling='auto' allowtransparency='true' onload='window.onmessage=function(e){var c=0;var o=document.getElementById("zoekframe");if(o.offsetParent){do{c += o.offsetTop;}while(o = o.offsetParent);};scrollTo(0,c);};' id=zoekframe>This text must be displayed by default.</iframe>
I have a fiddle on the following link: https://jsfiddle.net/mfhdwmya/

Answer 1

This question is awfully similar to Alternate text for iframes
The accepted solution for that one is:
<script>
$(function () {
$("iframe").not(":has([src])").each(function () {
var ifrm = this;
ifrm = (ifrm.contentWindow) ? ifrm.contentWindow : (ifrm.contentDocument.document) ? ifrm.contentDocument.document : ifrm.contentDocument;
ifrm.document.open();
ifrm.document.write($(this).attr("alt"));
ifrm.document.close();
});
});
</script>
I'd comment, but my reputation was not yet enough.