Passing URL parameters from cookie to iframe

I'm having Pardot forms in an iframe on Sitecore.

Sitecore is storing the UTM values in cookies.

Now I need to capture those values in the forms inside the iframe.

I have found this article https://help.salesforce.com/articleView?id=000317377&type=1&mode=1 on how to pass values from the URL to the iframe, but as the parameters can get lost if the prospect is clicking on another page before coming back to fill out the form, it would be perfect to find a way how to pass the values stored in the cookie to the iframe. 

Did someone already do that? Any ideas?

Thanks!

Tom