Hi Laurent
I've found a way to simulate what you suggested. Here's what you do:
Instead of inserting a link element into the page, you insert a code element, make it executable and then paste this code into the code editor:
<script type="text/javascript">
window.setTimeout(function(){
window.open("++path/to/thankyoupage (from site root)++", "_self")},
<a title="download" class="open" href="++URL to download++" onclick="thanks()">++Download now!++</a>
This will trigger the download on click and will then redirect to the thank you page (replace stuff inside ++ ++). Note however, that I had to use an arbitrary delay of three seconds since the download will not start if the response for the thank you page returns earlier than the one to the download. That means, that it is possible for the thank you page to be displayed without the download ever having happened.
Be sure to make the thank you page hidden, so that it doesn't appear in the menu.
Hit me up again if you have any questions.
BTW: I'll see if we can make this kind of redirect an actual feature in cmsbox. I like the idea but it's much too complicated for the user to do at the moment.
Cheers,