How to pass UTM parameters from the form hosted on Marketo LP to the destination page

For any other CTAs on our marketo LP we use this code to pass UTM values: <a class="block pass-query" href="destination-page.com" target="_blank"><button class="btn btn-orange">Register</button></a>

This isn't any kind of built-in browser or Marketo function. You must have separate code on your page that finds <a> tags with this class and rewrites the href.

If you want to add the current page's query string to the Thank You URL of a Marketo form, do it like so:

MktoForms2.whenReady(function(form){
  form.onSuccess(function(vals,thankYouURL){
    var thankYouLoc = document.createElement("a");
    thankYouLoc.href = thankYouURL;
    thankYouLoc.search += ( thankYouLoc.search.length ? "&" : "" ) + document.location.search.substring(1);
    document.location = thankYouLoc;
    return false;
  });
});

But I really don't think this is the best way to transport the original UTM information (having just worked with a client late last night whose analytics team was confused by this approach). Far better is persisting the original UTM parameters in a cookie, or at the very least relabeling the parameters as original_utm_whatever so you don't create extra GA sessions.